I have been researching for weeks... Classic Army was top of my list. By the way the 3rd from the top on the CA website is actually basically an M16, the carbine is on page 2 i think. It has a shorter foregrip and collapsible stock. It is good for outdoors/indoors, while the M15A4 is probably better outdoors and could cause you havoc due to its length in indoor games.
I have one on order and can't wait for it to arrive. I purchased the CA M15A4 Carbine. There are other brands out there like KWA and G&P. They each have their pros and cons. KWA has amazing internals and can take a LIPO battery, which means greater potential for a high FPS gun. G&P is known for its high quality metal body. In my honest opinion, the best combo would be Tokyo Marui internals with a G&P metal body. But going that route seems more difficult to implement and expensive. Also there is the ICS M4 people say its up to par with the CA. It has a split gearbox and metal body. It also has a forward release function to relieve tension from the spring and unjam the gearbox. The only reason I did not choose that gun is the lack of aftermarket parts due to its split gearbox design. The CA has a standard V2 gearbox therefore you can mix and match a ton of great internal parts. Also, its alot easier to start with a solid exterior AEG and swap out internal parts than to work the other way around. The foregrip on the CA is said to be of much better finish than most AEGS in its class which is a bonus. The earlier models of the CA were lacking reliability but the newer models are top class. Anyway if you need to know any information about the M4/M16 series and comparisons let me know I have researched beyond the limit about them.
After you get age verified hit me up and ill let you know where to buy your AEG, every day im realizing how big of a London airsoft community there is..