Cormac,
Many of the AC/DC adapters sold in the States are actually suitable for 120 or 240 VAC, even if they're not marketed as such. You just need to make sure to check the technical specs.
That aside,
here's a compact 240 AC/18 DC adapter, or
one with a variety of tips. You'll still need a North American to UK power adapter plug, but those are dirt cheap.
I use a BBE Supa-Charger to power my Analogman Mini Chorus at 16 VDC. It's switchable from 120 to 240 VAC. I've never tried running my chorus at 18 volts, but 16 does make a noticeable difference.
Of course, I could just be "hearing" all the money I spent to get those extra seven volts...
P.S. I just got back from Ireland. Dublin, then down to Waterford and Cobh, around the south coast to Killorglin, and up to Galway. If I was in your neck of the woods I coulda stopped in for a pint. You can't get Bulmer's on tap here in Tennessee!