The western coast of the UK is pretty wild as it faces the open Atlantic.
In winter cod are the main species - nice sized ones being 20 - 40lbs. Conger eel are fairly common - some as big as 110lbs. Also monkfish and skate grow pretty big.
In the summer Bass are the best fish to catch and also plenty of mackrell which are quite small. If it has been a very warm summer we can get Blue sharks coming into our waters - up to about 200lbs - although our waters are too cold for them to be any threat!!!
You can get all sorts in British waters - Sturgeons have been caught but belong to the queen - it is an old law and I don't think she would be too happy with a smelly fish landing on her doorstep!!!
An example of bonkers British laws - the bloke caught it in swansea - my home town - offered it to the Queen and was told to do what he liked with it.
Apparently you can catch them and kill them - do what you like - but if you sell it you are breaking the law!!!
As to freshwater stuff there is Salmon and trout along with pike and various others - not really a freshwater man myself!!!
sea fishing in the uk is thankfully still pretty much unregulated but freshwater is a different matter - all sorts of different seasons and licences etc needed!!!