i would plant a few strains m8 if you want them to grow during the natural season and flower in the autumn, i would use regular photoperiod seeds but you can try some autoflower strains too .. see which works out best for you
go for anything early, dont just go for indica, in the uk its hard to get anything finished whatever it is, and lots of indica has late budding growth spurts
to get any kind of yield, you need to get plants very big in veg, they will grow huge in the uk during veg
get them out early start of spring so they can veg till august ,
they will not make dense colas like indoor
so you need to spread the yield over many bud sites, hence the need for big bushes
an indica i had last summer outside afgan#1 got to the equivalent of week 5-6 if it had been indoor,
outdoor this was into november no chance of finishing
but the bud was ok it had a nice coating of resin, but could of been much better if it was an early developing strain
even sativa hybrids can be better if you find the right one, like an early superskunk thats ready indoors at 6-7 weeks, might just get to 5-6 weeks of maturity outdoor , skunks are not so hardy though i find bugs and mold tend to get a grip on them easier
next generations seeds do a few strains (avlon) (dynamite) (romulan) that are fast small indicas, based on grapefruit and romulan genetics, these things pack on the weight early and resin early and are very smokeable at 5-6 weeks
the alternative is to control the light hours outside, and flower them during the summer , then you can grow any strain you like to maturity
[video=youtube;S1WQpZmxv-g]http://www.youtube.com/watch?v=S1WQpZmxv-g[/video]
peace m8