Towel Rail Wiring

Use a cord outlet to do the change from cable to heat resistant flex and situate the FCU elsewhere - like outside the bathroom.

An ordinary flex outlet isn't allowed in Zone 1 or Zone 2 since it wouldn't meet the IPx4 requirement. To make a connection in those zones you'd need a IPx4 (or better) adaptable box and cable glands. Best to get the connection into Zone 3 if possible - i.e. at least 600mm from the bath - and then an ordinary FCU can be used.

Did the OP realy mean Zone 1 - over the bath?

An electric towel rail is not allowed in Zone 1 unless it "cannot reasonably be located elsewhere." If it is installed in Zone 1 it must be 30mA RCD protected. If it's fed from a RCD protected ring (or radial) circuit, itself fed from the RCD side of a split-load CU then then you're OK, otherwise an RCD spur will be required. (The RCD protection is not required for Zone 2 or beyond.)

If installed in either Zone 1 or 2 the rail must be suitable for those areas and should meet IPx4.

Don't forget the supplementary bonding, if required.

I'll take your word for it. However, I'd then say the towel rail itself isn't either, since the last one I saw didn't have a flex inlet that would meet IPx4.

I covered that by saying:

If in doubt the OP should check with the appliance manufacturer.

Wonder if sealing both with something like silicone would make it comply?

Not if you want it to pass a Part P inspection...