I'm going to make the assumption that England players who go to the Euros can't be involved, including Rooney. To make it easier, I'm making the assumption that the England squad for the Euros will be the squad for the Montenegro match, except with Stevie G and Wilshere in for Fat Frank and Adam Johnson and Rio in for Kyle Walker/Phil Jones. I'm also saying that you have to pick a couple of players from each of the 4 nations.
Goalkeepers
Lee Camp (Nottm Forest/Northern Ireland, over 23) - The goalkeeper is an area where you pretty much have to pick an over 23, there are very few young top class goalkeepers, or even just keepers who play, so you need to pick someone with experience. I've picked Lee Camp from Northern Ireland because a) Northern Ireland are shit and I can't see where else they're players are coming from and b) he's not that bad, and none of the countries are blessed with great keepers.
Defenders
Martin Kelly (Liverpool/England) - One of the best young defenders in the country, versatile, which is very important for tournament football. Probably will start every game.
Two of Phil Jones/Kyle Walker/Chris Smalling (England) - One of these will go to the Euros, the other two won't. Whichever two don't will be the starting right back and centre half, with Kelly being the other centre back.
Kieran Gibbs (Arsenal/England) - Not exactly world class, but a solid left back to complete a very English defence.
Midfielders
Gareth Bale (Wales/Tottenham) - If there's one player absolutely guaranteed to be on the plane, it's Bale. Bale is overrated, absolutely, but he's still one of the better players from the UK at the minute.
Aaron Ramsey (Wales/Arsenal) - Ramsey is better than Jack Willshere, by a country mile, and has spearheaded Wales' recent revival. A dynamic midfielder, and the creative force of the team.
Darren Fletcher (Scotland/Manchester United, over 23) - Fletcher is an unsung hero in United's recent successes. He just quitely gets the job done, while the flashier players get the accolades. In a midfield with two very attacking wingers and a goalscoring central midfielder, you need someone to keep it together. Fletcher would be my pick, and he would be my captain.
Alex Oxlade-Chamberlain (England/Arsenal) - This boy will undoubtedly be overrated by all. Watch him on a mazy run to nowhere to see why. That being said, he has a real talent for getting goals from absolutely nowhere, and that could be useful here. Giving Bale and Oxlade-Chamberlain free reign on the wings would be a handful for any defence, particularly an under-23 defence from footballing strongholds like Iraq. Who'd have thought Arsenal would provide three players for a young British team?
Strikers
Danny Welbeck (Manchester United/England) - He may go to the Euros, but it looks like Agbonlahor and Defoe have sorted their lives out, so I think that he miss out. Creative, strong, goalscorer. I can't quite put my finger on what it is, but I think he's a decent enough player and probably deserves to go.
Daniel Sturridge (Chelsea/England) - How is he not in the England team? The man has it all, and would be an asset to the British side. He can drop out to the wing if Chamberlain cuts inside, and you'd end up with a very fluid attacking side.
Subs
Alex Smithies (Huddersfield/England) - He's a young keeper, and he plays for an unbeaten team. About it really.
Lewin Nyatanga (Wales/Bristol City) - Versatile defender, good for cover.
John Flanagan (England/Liverpool) - Solid right back, plays for a decent team at a young age.
Chris Baird (Fulham/Northern Ireland, over 23) - Clearly, Northern Ireland are shit and I'm struggling for players, but I've also picked him because he can play almost anywhere. This makes him useful in a competition where you only have 18 players.
Barry Bannan (Scotland/Aston Villa) - Bannan is robust, and he's started to become increasingly a big part of Villa's midfield. He's also from Scotland and not totally shit, so he gets a look in for that.
Jack Rodwell (England/Everton) - I almost put Rodwell in the starting line up. I think you could make an argument that he's one of the best young defensive midfielders in the country, and he deserves to travel.
Marc Albrighton (England/Aston Villa) - I really rate him, and I think he'd provide good cover on the wings for either Bale or Chamberlain.
Connor Wickham (England/Sunderland) - We're yet to see what this guy can really do, but he looked good for Ipswich, and probably deserves to go.
David Goodwillie (Scotland/Blackburn) - He's looked much better for Blackburn than I thought he would. The sort of player who can pull unusual goals out the bag.
Reserves
Danny Wilson (Scotland/Liverpool) - A defender that can play anywhere in the back line. The only reason he's not in the main squad is because he plays more for Scotland than Liverpool.
Jack Collison (Wales/West Ham) - Another player who can offer cover for a number of positions, and who looks to be coming good again after a long period out with injury.
Craig Mackail-Smith (Scotland/Brighton, over 23) - This player is sluggish, lazy and doesn't seem to do much except score. Lots of goals. All the time. If he was under 23 he'd easily be in the squad.
Hal Robson-Kanu (Wales/Reading) - Nowhere near as good as Bale/Albrighton/Chamberlain, but a versatile player that could replace any of them or play up front if necessary.
David Beckham is a has been and a huge waste of both an over 23 spot, and a spot in general. You can't afford a passenger when you only have 18 players.