If oxymorphone (aka Opana ER and Opana IR) was available, I would HIGHLY recommend it as your next choice. The ER formulation lasts a TRUE 12 HOURS when taken orally, and even if you crush it and insufflate it, you are looking at 4-6 hours of pain relief and euphoria. The Opana IR tablets can easily be injected, and for most people, 10mg of oxymorphone in an injectable form would last 3-5 injections, each injection giving extreme euphoria and lasting 3-4 hours (a -morphone that has "legs"!). It is very fast-acting when taken orally as well, the effects being felt within 20 minutes of consumption. When insufflated, the effects come on over a 10-15 minute time-frame, and you can feel the euphoria building quickly. It is THE drug of choice for chronic pain (I am a CP patient as well, but in the U.S.), beating out hydromorphone, oxycodone, morphine, and even diacetylmorphine in both euphoria AND pain relief.
However, as it's not available in the U.K., at least as far as I know, the Diamorphine tablets would probably be a good choice as a breakthrough pain medication, as I assume they will "kick in" faster than a standard Morphine Sulfate IR tablet due to the dual acetyl groups. It's possible, though, that those groups are broken off via metabolization in the liver or by the acid in your stomach. If that's the case, then I don't see it having much, if any, advantage over morphine (unless you break them down and inject them). I would talk to the doctor about it, and see if you can get something like Hydromorphone-Extended-Release and then the Diamorphine tablets for breakthrough pain. I can see that being a very effective combination.
Good luck either way!