What is a good alternative to Sea Bass?
Depending on exactly the recipe you are using it in, I'd either go for a meaty white fish such as Cod, haddock, or the cheaper fish like pollock.

You may also be able to get away with something like mackerel, which isn't as white and chunky but is much cheaper.

You should be able to get all of these in a UK supermarket.

You could also try a fishmonger (either in the Supermarket or independent) - ask them what you could use instead.