Very far away from you but Chiltern Aquatics just off J12 on M1 has a very good selction of healthy plants. As for chains, I never buy from the well knowm Pets... shop. Arround Milton Keynes Dobbies and Frosts have decent plants but only shortly after delivery. Its best to ask your LFS when they get plants and if they can order what you need.
sorry! just saw that it says Yeovil on your profile. Im in East Anglia and get a lot of mine online, ebay mostly. Wuld love to find a plant exchange though. I throw tons of clippings and cuttings away that could go to a good home.
Plant exchange would be a great thing to see. I think eBay will be best bet for me. I like to see the plants first but I'm too far from any good stores my local store watermarque are OK but like I say not for plants and other than that I only have pets at home