In my area (Bristol and Bath)
Tile Flair stand out as being very "trade friendly" with very competitive prices on BAL, and Tilemaster Adhesives. The staff are generally excellent, and it is extremely rare for **** ups with ordering.
Bristol
Tile (now in St George Bristol) are also excellent with a wider range of adhesives and ancillaries. Collinsons Tiles (St Phillips Bristol) are another excellent source especially for stocking tiles that others don't, as are Avondale Ceramics in Longwell Green, Bristol (next door to Tileflair lol).
Best for cut price equipment (without buying online) are CTD, but they are not most accessible for me.
Independent
tile retailers tend to be expensive, and not really "trade friendly." I have been caught out several times, running out of adhesives/grout/Silicon in out of the way places and been charged retail prices masquerading as trade prices by the local independent. I accept they are in a difficult market, but they never get repeat trade from me after I feel I have been ripped off.
A final creditable mention for the nationals; Topps,
Tile Giant and CTD. Not always the cheapest, but (I work a 50 mile radius) they have bailed me out of some sticky issues when its worth the extra 10 miles to their shop to get their trade deal rather than using the local independent.
But at the end of the day, when I am supplying the adhesive and grout, I try and stock well up at
Tile Flair, and keep a ready stock of coloured grouts (why do people struggle to decide the colour until the morning of grouting?) and a few bags of adhesives and levellers.