How do I bathroom / kitchen / extension? etc.

Which 'resin'?

My choice would be driven by what the guarantee is on each option and what it costs.

Personally I'd go for whatever the no cost option is which is still covered by the loft guarantee

On my single story kitchen diner extension I went for fibreglass because absent anything happening to it which really fucks it up I can repair it easily myself. The kitchen diner does not have a ten year guarantee. If anything happens to it I'm likely on my own. On my loft it's a 3 layer felt system because that's what the loft company used and it has a ten year guarantee on it.

Felt is - for obvious reasons - what literally any roof person can deal with. It's difficult to fuck it up. Safe option. Doesn't last forever, but there's less chance of the fuckers who fit it to mess it up so badly that it fails prematurely. Looks shit and the slate dust / particles that runs off it is really annoying. Can last a long time if done right.

Fiberglass - looks good if done right. Could be repaired with off the shelf stuff. Can take footfall unlike felt so if you want an unofficial roof terrace... Easyier to mold in up stands for skylights / vents / flues etc. Ask your builder for evidence that they have done it before and not fucked it up. Probably better for areas where access is straightforward because pulling it all up when they inevitably fuck it up is a pita.

Resin? God knows.

Did they mention rubber - EDPM? Also looks good, kind of a half way house between felt and fibreglass. Can get fucked by branches falling on it but you just patch it like a tyre. The folks who sell it say it lasts forever but they are probably lying.