Peter,
If you want a single copy of Model Boats the best way is to buy one at a newsagent such as W.H. Smith. However, it is quite likely that there will no copies left from last month at the moment as the next issue (July) is due to be published on 7th June.
You can order single back issues where they are still available but this will of course mean paying postage on top of the magazine price.
Colin