If you get a HSE Medical in England it is valid for malta the other way is to use the dive centre in malta you will be diving with (Buddies is a good one)
http://www.buddiesmalta.com/ the local price is around £10
and a permit to dive without guides can also be applied for at the same time if needed if you contact the centre with the arrival date they will make arrangements to collect you show you the dive centre/ medical and drop your gear off if needed
Reards
Kevin