You'll probably have to start the plants off indoors before you put them outside. I've grown a couple of times outdoors with OK results but only because I started them indoors. You may get away with planting them outside but only when the weather gets warm enough like may or june; but planting this late reduces your growing time. It depends on the strain too, I grew Afghani which is a very tough plant (survives cold, rainy weather) but it's not worth chancing it if you don't know how tough your plants will be. An established plant will be able to take harsher weather much better than baby seedlings, seedlings wouldn't stand a chance I don't think. That's why it's best to start indoors or in a greenhouse, you can buy those mini, plastic greenhouses from Argos or somewhere, they work good. It's important you grow a strain with an early finishing time. My Afghani finishes flowering in about 49 to 56 days maybe a little longer outdoors. The weather in the UK starts getting really shit around late September (where I live anyway) you want your plants to be finished around mid October at the extreme latest. The buds do not cope with rain and cold too well, they just rot. I noticed you said you'd like to harvest in June/August- No chance, they only get going properly in June-July and they don't even think about flowering until about mid-late August. They only begin flowering when the days get shorter.

A few security tips-

Do not tell anybody, try and get some bad dirt on your friend and threaten to expose it if he/she tells anybody about your plants or threaten to kill him or something (just kidding about the death threat LOL!) Don't make too many trips to your plants, the more times you go, the more chance somebody will see you, only go when you really have to. Don't have a direct trail to your plants like don't disturb/flatten any bushes, these are your cover, if someone is out walking and they see a trail they'll walk down it. Always have an excuse for being there, take a football or something or walk the dog if you have one. Choose the time of your visits carefully.