The British-English phrase (as) dim as a Toc H lamp means dim-witted.
Its origin is explained in the text containing the earliest occurrence that I have found, that is, an article about the international movement Toc H, published in the Illustrated Chronicle (Leicester, Leicestershire, England) of Friday 20th 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:
The second-earliest occurrences of the phrase that I have found are from 50 years of Toc H, by Margaret Cooper, published in The Birmingham Post (Birmingham, Warwickshire, England) of Saturday 9th 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.
The phrase then occurs in 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 24th 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 . . .”