cbaddiley did you know that Voxi, Smarty and Three all include Switzerland in their European Roaming? I believe they all also cover the Channel Islands, and, the isle of Man in their plans/European Roaming, unlike giffgaff.
A Three PAYG SIM card and one of their Add-ons is particularly useful if you travel further afield as they cover destinations in their Go Roam such as USA, Australia, New Zealand, Singapore, and, Indonesia. There's usually no point getting an Add-on any bigger than £20 as you can usually only use 12GB in these destinations. You can use 15gb in Switzerland.
Check out the information on their website, Australia is given as an example below. You have to activate and use at least once before you leave the UK.
Ringing an Australian number while you are there, however, is actually cheaper on giffgaff at £1 per minute.