The British-English phrase (as) dim as a Toc H lamp means dim-witted.

The following from Soldier and Sailor Words and Phrases (London: George Routledge and Sons, Ltd., 1925), by Edward Fraser and John Gibbons, explains the meaning and origin of Toc H:

TOC H: The War name for Talbot House, Poperinghe, “Toc” being the signallers’ vernacular for the letter “T”. The original “Toc H” was opened on December 15^th, 1915, being named in memory of Lieutenant Gilbert Talbot of the Rifle Brigade, killed at Hooge in the previous July. A second “Toc H” was opened at Ypres in 1916. The purpose was to provide a place of rest and a social centre for all ranks; an institution equipped with a “quiet room”, library, attractive tea-room, a chapel, and a garden. Church of England in foundation and ideals, the resident chaplain and superintendent welcomed all new comers, of whatever form or belief, and the chapel ever stood open to all, regardless of creed, unobtrusively offering its restful consolation. Poperinghe, as the military metropolis of the Salient during upwards of two years, was visited by thousands of officers and men, for practically every one of whom “Toc H”, with its unique atmosphere and surroundings, proved alike a club and a home from home. [The writer of this entry, for one, well remembers tramping into Poperinghe one Sunday afternoon from Dirty Bucket Camp, and how at “Toc H”, with a book from the library on “English Cathedrals”, as he sat reading on the lawn, he experienced the first respite from war conditions he had been able to obtain for many months. For that he will ever be grateful for Talbot House]. After the Armistice, “Toc H” was transferred by its founder, the Rev. P. H. Clayton, to London, establishing headquarters as “Mark I” at All Hallows, Barking. There it maintains itself as an association pledged “to conquer hate and consecrate humour, its members drawn from all classes of society and devoting themselves to carry into everyday practice the straightforward standard of fairmindedness, unselfishness, helpfulness, and Christian outlook—termed the four points of our Compass”. Toc H. obtained its Royal Charter as a self-governing body in 1922. Its chaplains (padres) are of all Christian denominations. The Prince of Wales is patron, and its groups and branches have spread over Great Britain and are spreading over the Empire.

The earliest occurrences of the phrase (as) dim as a Toc H lamp that I have found are as follows, in chronological order:

1-: From an article about the international movement Toc H, published in the Illustrated Chronicle (Leicester, Leicestershire, England) of Friday 20^th September 1963:

Toc H, born out of the first World War, is still doing good work . . . and is celebrating its Jubilee with an appeal.
Its symbol is a lamp. […]
“Our lamps may have a small flame, but our spirit is bright as ever,” said Mr. C. Stevenson, Secretary of Leicester Toc H when the “Illustrated Chronicle” called in at the local organisation’s headquarters and hostel at 44, Princess Road.
“Actually, the saying ‘As dim as a Toc H lamp’ is quite accurate. A lamp is our symbol. We always have one lit at our meetings. But those old-fashioned oil and wick lamps only have a small flame.”
[…]
[The movement] began in the shattered Belgium town of Poperinghe during the First World War. The Rev. Tubby Clayton and the Rev. Neville Talbot, both Army chaplains, were asked to open a club for soldiers.
The club was to be inter-denominational and designed to provide an informal atmosphere where the men could relax. At first, the two chaplains were going to call it “Church House” but their suggestion received a curt military reply: “You’ll never get any customers if you have a name like that. Call it something else.”
The centre was named after Neville Talbot’s brother, Gilbert, who was killed in the first German flame attack on Sanctuary Wood. Both Neville and Gilbert were sons of the Bishop of Winchester.
Talbot House, refuge of thousands of war-weary British Tommies of all ranks and backgrounds, became in the signallers’ abbreviations of the time “Toc H.”
Since those days, the organisation has extended its activities, and there are Talbot Houses all over the world.

A Toc H lamp—© The Trustees of the British Museum:

2-: From 50 years of Toc H, by Margaret Cooper, published in The Birmingham Post (Birmingham, Warwickshire, England) of Saturday 9^th October 1965:

It is a movement which is difficult to define. […] It has […] been responsible for the cursory description of people whose wits are not all that strong—“As dim as a Toc H lamp.”
[…]
Small groups still meet regularly throughout the country and, what is more important, attract young people.
They hold their symbolic ceremony of the lamps at the beginning. But this is kept short. The lighting of the lamp, a replica of those used by the early Christians in the catacombs of Rome, with the double cross of Lorraine on its handle, is the signal for all to stand in silence. “The Silence of Remembrance.” Then having shone in the darkness, so to speak, the light is darkened again.
Just a short ceremony; anything more would turn Toc H into a senior Sunday school. Members do object to the “Dim as a Toc H lamp” simile, because their lamp is not supposed to be as bright as a 100-watt bulb, only a light shining in the darkness.

3-: From It wasn’t the bell being tolled that told the band it was Christmas, a short story by Michael Dineen about the Hale and Rowley Foundry Prize Silver Band, published in The Birmingham Post (Birmingham, Warwickshire, England) of Thursday 24^th December 1970:

“’Ark the ’Erald Angels Sing,” announced Albert quietly, blowing a “C” on his euphonium. The band grouped itself roughly round the lantern they always carried. Around them new semi-detached houses winked and glittered with seasonal window trimmings. But doors, hung with holly wreaths and red ribbon, remained shut. Mid-verse, Albert paused, tapped the unemployed triangelist [sic] on the shoulder and said: “Off yer go, kid. Throw ’em a salute and wish ’em a merry Christmas and ’old out the box like I told yer.”
Albert shook his head sadly before resuming his music: “Dim as a Toc H lamp,” he muttered. “And that’s not a bad idea, either. We’ll do the Toc H next, then the YMCA, then the British Legion, then the RAFA Club . . .”