Kitchen retile

I'm not a tiler and have no real experience so can't really help you with the fairness of the price but a bit more info on the job might help others with experience to give you a more informed opinion

-What sort of surface are the tiles on?
-If they are on plasterboard then this might need replaced or repaired based on how much damage is caused in ripping them off.
-When you say total job does this mean the price includes tiles, grouting etc? i.e. is the tiler providing the tiles and all materials required?
-Is there any mosaic work or anything like that to be done?

My best Guess:

A quick google search shows UK day rates of tilers to be £150-200 and for coverage £30-50 per sqm. Based on this for the labor only, £100 per sqm strikes me as a bit expensive but again, there could be other work involved there that they are taking into consideration?

For that area an average tile from B&Q seems to be about £15 per sqm so for 10% wastage that's ~£90 for tiles, you can pick up a bag of adhesive for ~£10, grout probably something similar. So that's roughly £110 for the materials (give or take some other sundries). If you take the upper limit of what google suggests are reasonable rates per sqm of £50 then that would be ~£275 giving you a total of ~£385. If the removal of the existing tiles is a simple job then £500 may be a bit on the high side but if there is repair work to be done prior to fitting the new tiles then £500 might not be too bad.