Hi Tom, in addition to the names already mentioned (GRS, KGR) I would add Chalk Garden Rail for a fairly wide range of accessories. I rely on P&S Hobbies in York for anything Massoth/mfx but not sure about accessories. However he is good at keeping his website up to date. For buildings Tony Green Steam Models carries Pola and Piko.
I would also consider looking at EU websites especially Modellbahn Kramm, Modellbahn Lippe and recently Modellbahndiskont is Austria. My experience recently (after some post-Brexit disruption) has been good, expecially with items below the GBP140 cutoff. On those they will zero-rate for export and I haven't so far had to pay UK VAT on these imports. Even over that amount If you are after something unavailable in the UK the end price is fair. As you are in Jersey you should get the zero-rate benefit and (I assume) a mere 5% GST?
Regards, Adrian