They will be going onto a concrete floor which has under floor heating installed.
You don't say if it's a wet or wire system? Just tile over if wet. If it's a wire system you'd be wise to lay an SLC over it to protect from tiling & to give you any chance of repair if it ever goes wrong; it also provides a flat tile base which is important if the tiles are large format. You should also consider laying the element over insulated tile backer board rather than directly onto the concrete screed, it helps direct heat upwards through the tiles rather than it being absorbed by the screed & reduces warm up time.
Am I right in saying the tiles have to be sealed before laying them and again twice after grouting?
Yes, seal once before laying, clean & seal again before grouting & then seal twice more after grouting.
Do I have to use a specific adhesive and grout as the floor will be contracting from the ufh?
Assuming your doing the tiling; you must be careful with adhesive & grout application to avoid staining, don’t slap it around! Use a quality trade flexible cement powder adhesive & grout, cheap own brand & DIY materials are mostly crap. Because of the risk of bleed through staining use a Flexy Rapidset adhesive to reduce risk but it has a short pot life so don’t mix any more than you can use in around 35 minutes & clean your bucket & tools thoroughly between mixes. Use white adhesive rather than the standard grey for light coloured travertine, again to reduce risk of staining. Same applies to grout; don’t be tempted to use a dark contrasting colour. No less than 4mm on tile spacing & if large format 5mm; you will need a decent diamond wheel wet tile cutter.
