"i wish muvver had bought white mice" is assumed to be an original by Mabel Lucie Attwell.  Although slightly different to the final image found during my research, it possesses all the characteristic, identifying features and typical qualities associated with her works.
I think this is likely a preliminary version, as the rug shown in the image is at an awkward angle (the perspective is wrong) so she may have redone the piece. 
The board it is painted on, Whatman's Sketching Watercolour Board, is approximately 10.5 inches by 7.25 inches (IMPERIAL)  is in keeping with an early 20th century art piece. There is some fading and some foxing.
 
Mabel Lucie Attwell, SWA (1879-1964) developed her own imaginative, and often amusing, imagery through annuals and postcards. Then, as her popularity increased, she applied it to a wide range of products. She was a household name by the 1920s, by which time no home was complete without an Attwell plaque or money-box biscuit-tin.
The daughter of a butcher, Mabel Lucie Attwell was born in Mile End, London, on 4 June 1879. She was educated privately and at the Coopers’ Company Coborn School, in the Bow Road, where she showed an early talent for art. She funded her own studies at Heatherley’s and St Martin’s School of Art (1895-1900), and met her future husband, painter and illustrator, Harold Earnshaw, while at the latter. However, disliking the formal training, she completed neither course, and turned with some relief to contributions to periodicals and commissions for illustrated gift books. Influences on her work included her friend, Hilda Cowham, and such members of the London Sketch Club as John Hassall and William Heath Robinson. After her husband lost his arm, through active service in the First World War, she became the mainstay of the family. In 1922, she produced the first of her hugely successful children’s annuals. Three years later, she was elected to the Society of Women Artists. A rapid and prolific worker, with a keen business sense, Attwell also designed cards, posters, calendars, figurines and wall plaques, and saw her creations turned into crockery patterns and soft toys. For most of her career, Attwell lived between London and Sussex with her husband. Following his death (in 1937) and bomb damage to her two London homes (in 1940), she moved to Foxfold, Wiltshire. At the end of the Second World War, she settled in Fowey, Cornwall and remained there until her death on 5 November 1964.