I don't know about the UK, in America the standard bearer is Liveaquaria mostly because they have been around for a long time and have long offered quarantined fish, which is almost non existent in the rest of the industry.
Still I am not a huge fan as a good petstore is always going to be the best option for fish. I know it is tuff with covid but there are two very good fish stores in my area that have been around since 2000 and still have a fairly good selection of fish. Those two things are related as it is the stores that are able to keep their fish alive which buy way more fish than they can turnover in months so I would encourage you to look further and you should find what your looking for.
Still I am not a huge fan as a good petstore is always going to be the best option for fish. I know it is tuff with covid but there are two very good fish stores in my area that have been around since 2000 and still have a fairly good selection of fish. Those two things are related as it is the stores that are able to keep their fish alive which buy way more fish than they can turnover in months so I would encourage you to look further and you should find what your looking for.