ooh, we were going to do get encaustic tiles for our bathroom but a long story and shrinking budget later have had to abandon the plan. However, I did end up contacting a bunch of these places so here was my experience.
One guy at Habibi in wandsworth bridge road was very nice, but totally misquoted us time and costs for the tiles we want. They were supposedly in stock in Kensel rise and when we went up there to pick them up (over an hours drive from our house - fun!) not only did they not have them, but the guy was incredibly rude about the whole thing. Kept telling us to buy a different colour zellige instead as it was close enough. (It so wasn't.) Kept us waiting for 15 minutes while he chatted no the phone, etc etc. I was one of the most frustrating days of my life, and really put us off them.
We called Dar Interiours a number of times and emailed and never heard back. Even went to the listed address which despite being in Fulham was sketchy sketchy. And no sign of them there.
Emery et Cie have gorgeous tiles and a show room at Retrouvius in Kensal Rise (just around the corner from Habibi which unfortunately we didn't know so we ended up doing too trips!) What we wanted was going to end up being about 200 pounds a square meter which we just couldn't justify.
One other company that is worth trying is a Spanish company called Mosaic del Sur. They have a bunch of stock and that can be delivered in about a week, and the made to order stuff is 6 weeks.
There are also the Olde English style victorian geometric patterns which are unglazed, and those you can get more places, but where we've gotten them is Criterion Tiles on Wandsworth Bridge road.