I really don't rate Swallows. I live near the Rayleigh one which is huge (Their main one IIRC), but the actual diversity of fish they stock is pretty dissappointing. On the FW side they have little more exciting than livebearers and corries with the occasional cypranid shark.
The only other fish they seem to get in are RTC and Clown Knives.
The marine side is ok with some good stuff, though their two largest display tanks are pretty bad. The FW has a RTC in a tank that has a footprint no bigger than 6x5 (and the cat is now a little over 2 feet) and the marine display has a Leopard (or tesselated) moray which can end up huge, as in needing 1000 gallon tank huge (10 feet is attainable in the wild).
It used to stock some interesting fish, but it seems that rather than educating the staff about more fish, they just cut back to the bog standard fish.
Furthermore, I find a lot of the staff are either dismissive, or boorish, in their approach to handing out information.
I am becoming more impressed with Wickford Aquatics as it has a huge diversity in fish and has some real rare things, though it does also have a tendancy to stock Mbu puffers and RTC, though some of the customers do have huge tanks (one has an 8x3x3 and another one around 15x6).
Sadly the best one round here for FW - Shotgate - shut down last year due to the rising water costs. Thankfully I have a good relationship with a small independant one near me and he will go to the wholesalers and try and get whatever I want.