It's in the JSP 886, thanks Gray.
Basically, as I thought, coveralls and other protective clothing, Other Ranks No 1 SD, No 6 SD, No 2 SD trousers, slacks and skirt, jersey, tie, General Purpose Jacket, Combat Soldier 95 jacket and trousers, wet weather jacket and trousers and SD caps are all are authorised to be laundered at public expense.
So I have to wash my own shirts and socks, which I do. Not bothered about that, no hassle at all, but if I can get a lump sum of £72 for the price of a stamp then I'll give it a go. Not a huge amount of cash but let's be honest, if you knew you could get £72 just for sending letter you'd be mad not to do it, you can even get the letter from MSE forums! Now I have the JSP I can quote it in a letter and it should be clear to HMRC what services are provided.
With regards to machines in the blocks etc, I live out. Plus any provided by SIF are not free, I would suggest in that case there is no laundry provided by the RAF.
Done some searching and found the previous posts. Mutty was correct, back to 2007, it was discussed and the conclusion was made after a letter back from HMRC stating that:
"HMRC also confirmed that no tax relief was available for Servicemen for the washing of their uniforms: officers, because they receive recompense for ‘upkeep’ within their tax allowance; and airmen because they have the option of having clothing laundered at unit level."
Well, IAW JSP 886 shirts and socks are not covered, therefore the Service does not provide laundry for all items of clothing. I will be pointing this out to HMRC. Going to give it a go anyway, no hassle in trying.