If you could give us an idea of what your maximum budget is (including batteries, charger, etc.) then it would be easier to give you some recommendations. As the others have said - the G&G Combat Machine is a great budget rifle but if you have more to spend then obviously your options increase as does the longevity/quality of the rifle. If the £150 mark is your ideal limit then the ICS Sportline M4 is the budget rifle I'd recommend personally as it's just as good as the G&G and can be brought quite cheaply.
The main thing is to find a style of rifle you like (e.g. M4, AK, AUG, G36C, etc.) and then go from there.
By the way, welcome to the forum