The problems with this are:
1.) You need a banking license to take deposits.
2.) The website might start off as full reserve banking but be tempted in to fraction reserve banking years later.
3.) Running an exchange at 1:1 would lose money.
4.) It is centralised and open to attack.
5.) the issuer of poundcoin will have to be trusted.
1) You need an e-money license, which is close to a bank license but not the same. A few companies in the EU hold one of these, they don't really sound like bank names though.
2) True, though in my vision this company would have a fractional "hot wallet" and lives from interest created on deposits that are deposited at very conservative funds/accounts. One could also make these public. Having 100% coverage is a requirement for e-money providers I think.
3) How/why? I think paying the actual costs as fees would only be fair and is also allowed accouding to e-money regulations.
4) True, both alt-chain and colored coin approach are risky and reputation is at stake.
5) True as well, kind of. Poundcoins would not be different from GBP, but there's the problem that if you allow anyone to issue them that anyone can do fractional reserve. Exactly because of this there are so many hurdles in creating a bank and getting a license for that, to make sure only few people/institutions can do this legally. E-money providers are not allowed to do that btw.
All in all I also already did a bit of research (though rather with the altchain than colored coin approach) but as you I see that there are serious hurdles in building confidence. Having an official license might help though.
About that "everyone can issue Poundcoins and you hav to trust these" - that's rather the concept of Ripple if I understood it correctly and it should work because you owe people you personally know some money, not some strangers owe to you.