You have already narrowed down your choice of regulator/system.
The Melee is the only one (complete that is) that you can get with an HSE bottle so given that you are in the UK you don't have the luxury of a second choice, not unless you spend more money, but you are saying you are short on cash so the Melee will be the way to go.
Given the above if anyone can be sure that the Pure Energy regulator fits an HSE bottle post away, as far as I am aware the only way you can do it is to put a thread adapter on.