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The British-English noun Mrs Mop, also Mrs Mopp, designates a charwoman, a cleaning lady.

This noun occurs, for example, in the following from Hassle-free spring clean, published in The Bolton News (Bolton, Lancashire, England) of Saturday 15^th April 2017 [page 20, column 3]:

The days are getting lighter, we’re all feeling a bit more social, planning holidays, freshening up our wardrobes and, erm, getting stuck in with the Big Spring Clean?!
If the mere mention makes you shudder, don’t despair—it’s not all about putting on a pair of Marigolds and having to sacrifice a precious weekend to play Mrs Mop.

These are, in chronological order, the earliest occurrences of the noun Mrs Mop, also Mrs Mopp, that I have found:
—Note: This noun was popularised in 1942 by the charwoman’s name in the BBC radio comedy series It’s That Man Again (abbreviated to ITMA), broadcast from 1939 to 1949: Mrs Mopp was interpreted by the British comedian Dorothy Summers (1883-1964):

1-: From an account of the meeting of the Swalecliffe with Chestfield Women’s Institute that was held on Wednesday 14^th August 1940, published in the Herne Bay Press (Herne Bay, Kent, England) of Saturday 24^th August 1940 [page 6, column 5]:

After tea had been served an amusing sketch entitled “Mrs. Scrubb and Mrs. Mop” (Mrs. R. Jones and Mrs. Cooper), was well acted, and members showed their appreciation by their applause.

2-: From Behind The Scenes In Radio Variety, by Jean Merrill Du Cane, published in the Evening Chronicle (Newcastle-upon-Tyne, Northumberland, England) of Monday 16^th February 1942 [page 2, column 2]:

Grinning like a satyr, Jack (Funf) Train reads his roughly-pencilled script. This little tea party in a B.B.C. canteen began as a conference—now the teacups and crumbs are swept back and Tommy Handley, Ted Kavanagh (the man who writes the script), producer Francis Worsley, Dino Galvani (Signor So-So) “Mrs. Mop,” and all the rest of the “It’s That Man Again” gang are at work.

3-: From the column Town Topics, published in the Bolton Evening News (Bolton, Lancashire, England) of Thursday 19^th February 1942 [page 3, column 6]:

MIMICS
The keenness with which children listen to radio items is often revealed by their clever mimicry. A school teacher in Bolton admits her astonishment at the gifts shown by some of the boys in her class. She decided to fill up an interval that would normally have been spent out of doors in acting plays, and left the selection to the young people.
“From the boys,” she says, “I got ‘Itma’ to perfection. I could close my eyes and hear Tommy Handley and his satellites, even the [sic] Mrs. Mop the Corporation cleaner, almost as if I’d switched on my wireless set at home.”

4-: From Jottings of the Week, published in The Falkirk Herald and Scottish Midlands Journal (Falkirk, Stirlingshire, Scotland) of Saturday 28^th February 1942 [page 4, column 6]:

And now we must stop. We mustn’t say anything about the weather, you know. The editor won’t even let us have a right good swear at it! So, as Mrs Mop would say, “Ta-ta for now!”

5-: From an account of the meeting of the Olney Women’s Institute that was held on Wednesday 25^th February 1942, published in The Bucks Standard (Newport Pagnell, Buckinghamshire, England) of Saturday 28^th February 1942 [page 3, column 5]:

The social half-hour was undertaken by those members whose names begin with A, and was most successful and amusing. […] The other part of the programme was impersonations in costume of B.B.C. familiar figures. These were exceptionally realistic and were greeted with loud applause; they were Grandma Buggins, Mrs. Mop, Enoch, Old Ebenezer, Henry Hall, Suzette Tarri and Syd Walker.

6-: From an article about the 1942 Charities Week Campaign organised by the students of Aberdeen University, published in the Press and Journal (Aberdeen, Aberdeenshire, Scotland) of Tuesday 21^st April 1942 [page 3, column 4]:

News of Gala Week has reached Tommy Handley, the radio favourite, who has sent a personal message from himself and his “Itma” company.
“Just a line to wish your Charities Week a big success,” he writes. “All my company send their wishes too. The ‘Diver’ wishes to be remembered. ‘So-so’ says he hopes you get lots instead of ‘nothing at all—nothing all.’ Mrs Mopp would like to ‘bring something for you.’ And Sam hopes that nothing terrible will happen.”

7-: From the caption to the following cartoon, published in the Bolton Evening News (Bolton, Lancashire, England) of Saturday 25^th April 1942 [page 3, column 2]:

CAMEO COMEDY

He (who has breakfasted well): “I’m afraid I’ve eaten all the porridge, Mrs. Mopp.”
She: “Oh! That’s all right, sir. If you ’adn’t eaten it the cat would ’ave ’ad it.”

8-: From the column Week-End Wireless, by ‘The Listener’, published in the Yorkshire Evening Post (Leeds, Yorkshire, England) of Saturday 2^nd May 1942 [page 2, column 1]:

We said good-bye, for a season, to Tommy Handley’s “Itma” last night (recording to-morrow). When I ventured, a few weeks ago, to remark on the popularity of this show, I was soundly abused (anonymously, of course).
Since then it has received the Royal command—the first radio programme that has ever been so honoured.
In addition to Tommy himself, the chief characters in “Itma” are Jack Train, who is Funf, Claude, Lefty the gangster, Mr. Bookham, the theatrical agent, and the oriental who jabbers on the phone Dorothy Summers is Mrs. Mopp, Clarence Wright is the cheerful commercial traveller, and also the man with the laugh. Fred Yule is the tramp Dino Galvani is Signor So-So. Sydney Keith is Sam Scram, who tells the Boss that something terrible’s happened. Horace Percival is Cecil and also the traveller in postcards and scented soap, as well as the diver.

9-: From an account of the show given by the 4^th Streatham Girl Guides and Brownies on Saturday 2^nd May 1942 at the Holy Redeemer Church Hall, Streatham Vale, published in The Streatham News (London, England) of Friday 8^th May 1942 [page 5, column 3]:

The versatile Guide Captain and two Guides gave a comedy song, “The Cleaners,” in appropriate “Mrs. Mop” costumes and accessories—a most amusing item.