I just checked the weather for Leed, England online. Highs in the mid 60s and lows in the 30s (farenheit) right now, is that correct?
I see no reason why you couldn't begin to accustom 5 week old chicks to living outside. Perhaps you could try taking them outside on nice, sunny days to spend the day in their coop and bring them back in at night for a week or so. What you really want to watch out for is drafts.
The general rule of thumb is 90 to 95 the first week, decreasing by 5 degrees a week until such time you are down to 70 degrees (end of week 5); at which point they should not normally require supplemental heat.