Hey. Welcome to the site.
Are you going for a defence first so that you can order an RIF or just 2-tone? Given that you only need to wait another month and attend one more game after the one you're going to this week, I'd say getting a site membership and UKARA may be the better option. Not sure if you're going for that or not.
So the general rule with airsoft is that barrel length doesn't mean a whole lot. You
do generally get more FPS from a longer barrel, but equally that can be done with shorter barrels and with AEGs is mostly dependant on the cylinder and spring you're using anyway as longer barrels need a larger volume of air to propel that BB. Accuracy tends to depend on your hop and the quality and/or width of barrel rather than length. For this reason, MP5s and M4s are not intrinsically different when it comes to their performance. What matters more is the manufacturer and any aftermarket parts you've added to it (such as a new barrel, new hop unit/rubber).
If you're just starting out but are happy with spending that much money then my best advice would be to drop the pistol and put a good deal of that money (say £180-250+) into a rifle then spend the rest on some gear, ammo and all that stuff. Pistols are fun, sure, but they're pricey and really you very rarely get to use them (especially when playing woodland). May be something to consider.
As for what brand to get: ICS and G&G are well recommended as both entry-level and mid-tier guns. If you have a larger budget you can obviously get something that performs slightly better, but airsoft is all about how much you're
comfortable with spending. You don't need to spend £400 on a single gun to get something that's going to compete well.
This guide explains some of the starting-out stuff and touches on UKARA and defences too.
This index of links may also help.
In the end it's up to you. If you want to spend £400 on both a pistol and a gun then consider an ICS or G&G M4 and a WE pistol. That should leave you with up to £150 spare for everything else. As I said though, pistols aren't entirely necessary. If I were starting out today I'd shop around and find something I really like the look of for £300 like a Krytac M4. That said, AKs, the SA80 family, SMGs like MP7s, MP5s etc. are all viable options if you get the right brand. What do you like the look of? Have you tried anything out when you last went? It may be worth handling some other peoples guns when you go this Sunday and see what you like the feel of - people are usually more than happy to let you try them out.