later - The answer appears to be that they are stronger than straight American beer, and a little stronger than most British beers, at around 8% alcohol.
So I've written the (minor) scene as follows:
Thor downed his third pint of Theakston’s Old Peculiar and said “Another!”
“Are you sure?” asked Darcy. “That stuff’s stronger than the beer we got in Puente Antiguo.”
“It is mild compared to the ales of Asgard. Or even the makers of boilers I drank in your land.”
“Your funeral. This stuff sneaks up on you.” She waved to Ian, who was queuing in front of one of the beer tents, and shouted “make that three more!”
“Three?” said Jane.
“One for me, of course.”
“I am not nursing you through another hangover.”
