How do I bathroom / kitchen / extension? etc.

There are contractors who specialise in retrofitting wet UFH. They use a machine kind of like a walk behind cut-and-break saw to cut tracks in the floor which the pipes are then laid into. You could DIY it either if you're handy and have the time but it is a rather messy job.

@Nahguavkire you did this DIY right?

Can you remember what the rough p/m² cost was of installing wet UFH?

Also in terms of how it connects to the boiler, is there anything special about it? Ie would a plumber just be picking one of the hot pipe and return feeds from an upstairs boiler and connecting them to that?

Cheers.