Mirror Quartz tiles (Galaxy, Lightburst, Gulfstone) can be atrocious from online dealers. I would expect to pay £50-60 minimum for a good quality. Make sure you can get physical samples before committing to any serious orders.
We've had reports of some of the cheaper knockoffs having some serious issues. Certain batches when exposed to sunlight or prolonged use of UFH, the resins & fillers have had adverse reactions to the temperature changes, changing colour, clouding, crazing and cracking.
Topps tiles quality are pretty good I've heard from customers and have seen some samples, they're our biggest local competitor, both physically and online, so I only give them credit where it really is due. If their quality is good here, I would hope it's equable nationwide.
, whoever you buy from, try to find out who their supplier is and do a bit of independent research to make sure they're good. And get samples!