Many seed companies advertise heat tolerant lettuce varieties. Renees Garden offers quite a few; perhaps there is a UK company with similar offerings? From my past experience Jester is very good for summer growing, and a tiny butterhead called Tom Thumb has been exceptionally heat-tolerant in our garden too. Evidently butterhead (aka bibb lettuce) is one of the best kinds to grow in summer. And I feel your pain on the slug front. I hand-picked 3000 slugs in May; after that I went out and bought a jug of slug bait too. There is one lettuce that I'm aware of that seems to be resistant to slugs: Alkindus. It's a pretty red butterhead, and last fall the slugs attacked several varieties of lettuce that I planted out-- but they left Alkindus alone. It also appears to be slow to bolt, which is another good quality.