I pay a bit extra to have the value date "0" (same day), so that's not a problem.
It's a pity that you have Barlcays export them next day, and only HSBC Hong Kong the same day, because a USD wire transfer to Hong Kong costs me 5 times as much as a GBP wire to UK - I pay only 4 GBP for a wire to UK, but it would cost me 30 USD to wire to Hong Kong (both prices same-day).
Now you have a relationship with HSBC, couldn't you open a HSBC account in GBP in UK so transfers there could also be credited instantly? And for that matter HSBC also has an EUR branch in France, why don't you open an account there? Once you have one HSBC account, you can easily have accounts in all their other branches in other countries.
I could open a HSBC in Europe myself and transfer money same-day there, and then also same-day to your HSBC UK, but to convert EUR to USD in HSBC would cost the 2.5%, while transferring GBP to Barclays the currency conversion is free (I can convert EUR to GBP for about 0.2% so that's not a problem).
Couldn't you open EUR and GBP accounts in HSBC (any branch, may be even Hong Kong), and there do the same thing as you do in Barclays - credit MtGox accounts with 0% currency conversion?