Hello Sid welcome to uk420 well obviously your too late to start this year, if i were you i would spend my time doing a whole lot of reading, so many strains out there its hard to just pluck out the right one for you , same goes for compo do you want to go organic or does it not matter ? , alot of growers here prefer allmix but many others use different ones successfully, good reports have been had from J.A.B. , Westlands ,BnQ multi to name a few, i think an early strain might suit a novice for a greenhouse grow but there are many others to choose from, certain strains are just not suited to natural uk growing without alittle help, chiefly in the latter stages of flower, do some research on the different outdoor strains available and see if theres one that takes your fancy, do some reading in knowledge base and grow diaries to get a feel and understanding of what you want to do, it may seem like there is alot of information to take in at first, but with time and patience you will understand better and hopefully armed with knowledge you can produce a successful crop
Happy reading
Rj