Growing indoors with greater success?

The key to growing tomatoes outdoors in the UK is to pick early and/or blight resistant varieties. You will get most success with cherries and medium sized fruit. It's harder to get beefsteaks to ripen outdoors unless we have a really good summer or if you can grow them near to a sunny wall.

There are new blight resistant varieties coming out every year now and the flavours are fast improving. Since most of them are hybrids, the seed is more expensive, but most of these can be bought for a very good price at PremierSeedsDirect:

Cherry - Crimson Cherry, Crokini, Rubylicious
Medium - Crimson Crush, Cocktail Crush
Plum - Crimson Plum (Nagina)
Beefsteak - Pink Honeymoon, Buffalo Sun, Rose Crush, Burlesque, Oh Happy Day
Bush - Lizzano, Losetto, Consuelo, Romello, Summerlast, Koralik

Most pepper plants will not get very tall so I think a polytunnel is unnecessary and it would be more cost effective to build some cheap low tunnels, which you could also grow shorter bush tomatoes under. This chap I watch on YouTube grows most of his peppers under low tunnels with tremendous success, you can see how well they do in this bit of video - https://youtu.be/7EaELed2EsI?t=385