If you live in Cardiff or are just visiting, there is a small town called Penarth you can go to. It mostly has attractions but there are a few good sights for birdwatching.
Too summarise, every common garden bird can be seen here. If you need a place to find a really common bird that you just haven't gotten a picture of yet, this is the place. Full of starlings, house sparrows, goldfinches, great tits and blue tits, aswell as 4 pigeon species, 2 thrushes, house martins, swifts and a manor of gulls. The coast brings around 7 gulls, with a few rarities.
At the pier, you can see the gulls. In the summer, I have seen yellow legged gulls and even a caspian gull. Unfortnately, the amount of people deters smaller gulls, so Penarth is dominated by anything between the size of a black head and great black backed. Windsor gardens attracts blackbirds, woodpigeons, goldfinches, sparrows and other small birds. Even though I haven't seen one before, there was apparently a greenfinch there.
On UpperCliff Close, house martins and starlings can be seen in the summer. Collared doves and song thrushes are also here.
Overall, just a place to catch up on any usual birds you don't have a photo of for some reason, whilst also seeing gulls.