In reply to SenzuBean:
The reference for a belay jacket for Scottish winter is the Mountain Equipment Fitzroy. Mid-level warmth, sized to fit over everything, synthetic so moisture is less of a concern. Most brands do an equivalent.
The bombproof belay jacket refrences are the Patagucci DAS parka and ME Citadel. Warmer, heavier, bulkier, again synthetic but will feel like the best purchase ever if the weather is foul.
After these you get the synthetic vest, down jacket etc options which are less standard but equally valid.
These decisions are best made by trying them all on and seeing which you like best. If you can’t, most manufacturers have a note on their website about the type of fit, i.e. standard, slim, oversized etc. It helps if you know how that brand sizes, ie – Patagonia is aimed at chubby yanks, Rab for the slim etc….
P.S. If it’s your first trip, will you actually be belaying much? If you’re climbing grade I/II routes they are often soloed (large snow gullies - protection is often sparse and the climbing quite easy) and I get pretty hot whilst climbing them (moving over snow/neve/ice is quite an all body workout and does get the core temperature up!)