Unfortunately my local fish stockist are only pets at home however the Japanese koi carp company is not too far for the occasional visit. If anyone knows of any good stockist (I am in North Herts but am willing to travel to any that is not too far)
Without a city for reference I can only suggest you run a Google Maps search on "Aquatics Hertfordshire". I did that when I got started and visited all within a decent traveling distance.
Maidenhead Aquatics have 3 stores in your general vicinity, St Albans AL2 1DL, Kings Langley WD4 9LH, Milton Keynes MK17 8UF. I use my local one near Wigan almost exclusively, despite it being a few miles further than my very local PAH store (which I only buy dry goods from).
As Sue has said, don't take what shops say to you as gospel as most contradict each other. Always remember that a shop is there to sell, so don't invest in spending cash unless you are certain you know why you are doing it.
To be honest, some of the shops I've been in would try to sell sand to an Arab if they could, but that's another story.
Best of luck, we are all keen to help where we can.