I'd love to find out how big the magento 2 dev community is in the UK. Approx how many dev houses and who they are...?
There are many Magento agency in United Kingdom. magento is large scale product and Their community is famous for contributions in magento.
You can find more details on Magento Websites,
You can filter for getting the specific result from above url and many smaller agency that are not listed in above links also available.