Thanks, I just got a reply from DSX on the subject too. Apart from anything else, they point out that the market is so small it’s not a big deal for them. Firstly, it’s chicken and egg. There is a demand in the UK, which the banks and our isolation from the euro have caused. Without access, demand is stifled, so no-one opens a market, hence the observed market is small.
Secondly, they maybe a UK based exchange, but they are unable to offer a service that can take GBP natively using currently existing systems. These layers of fee charging and complexity are what keeps the market small, much to the banks amusement I suspect.
They did point out that HSBC only charges £4 for SEPA payments though, so I may investigate further