I cannot contribute with information about this specific clock, but would say that GBP 20 for a nice looking clock, which is working properly, is very well bought. Compare what else you get for that, e.g. 2 pizza and perhaps a drink on top.
You will find a lot of information on Enfield clocks here on this site, if you use the search icon at the top of the page.
They are not valuable clocks, they were made in their thousands in the 1930s to1950s and were commonly seen in homes everywhere. Often known as 'granny's mantel clock' because everybody's granny seemed to have one.
You have paid about the right price and if it's working well, then you have got a good deal.