If you want to emulate British BBQ cooking, you'll have to get a few essentials right first:
1. The person doing the cooking must be male. All British men know that a woman should never be allowed anywhere near a barbecue.
2. Said male must be heavily intoxicated.
3. The cook must not wear a wristwatch and all clocks must be removed from his vision. This is so that he can remain blissfully unaware that people are still waiting for their first taste of food over 3 hours after he said it would be ready.
4. There must be a large number of pets and children running around the cook. This is so he will have someone (or something) to swear at periodically.
Having got those points sorted out, it's now possible to address your question:
Direct aforementioned British male to a branch of Iceland and invite him to select a turkey. He will, of course, make this selection based upon the principle that 'biggest is best' This will be irrespective of the fact that most of the guests for Christmas lunch will either be vegetarians or will simply hate turkey
Now, here's the really important bit. Take said male to the nearest cookware shop and direct him towards the section labelled 'Kitchen Gadgets'. To any British male, gadgets are like gods. They aren't meant to be practical; their primary purpose is to be worshipped.
Draw the attention of said male to the existence of those items labelled as 'Chef's Blow Torch'. Your problem is now solved. You're guaranteed a turkey which will be totally frozen on the inside and completely black on the outside ;-)
Chris