I am originally from the UK growing up in Merseyside and North Wales area, and I enjoy my birding trips back to the UK. There are also some great spots in Europe to explore.
There is good birding in London at the Wetland Centre, with both a display of birds in cages and ponds, and wild birds on open lakes that can be viewed from hides. Good selection of waterfowl, and rarities turn up.
The main attraction for UK birders here is the (Red-billed) Chough. The scenery is also very nice.
Another great spot for waders, with plenty of species on our day counts including Avocets. There are many good birding locations around the Norfolk coast, but it is also important to choose the right time of year for target species. There is a big difference between summer and winter species, and the passage migrants. We stayed at the White House Farm in Hindolveston who provide great accommodation near the birding areas, and are very birding friendly. Highly recommended.
About 1 hours drive south down the coast from Barcelona, the natural Ebro Delta is a wonderful place for water birds and shore birds. We got about 100 species in a morning. It is warm and sunny and birding is easy. Plenty of Flamingoes here.
You don’t need to go any further than the capital of Portugal for some great birding. We had a short tour with Birds & Nature Tours Portugal and it was very valuable having the knowledge of a local guide. There are many waders and water birds, plus raptors and passerines species. We got about 80 species in a day here, including some good views of Bee-eater and Hoopoe.