I don't know about MOD, but NHBC standards now specify three prong bayonet fit low energy bulbs (on new builds at least) that are very difficult for consumers to source. When I was still renting, last year, I went into B&Q and spoke to a chap in their lighting dept. He told me that they don't stock them yet, and couldn't tell me when they were likely to because they were so damned expensive and people were unlikely to buy them.

I eventually got a price for them, elsewhere. £15 each for 75 Watt equivalent bulbs.

Unsurprisingly, and with the landlady's permission, I changed the lamp holders for £1.43 each and used regular two pin energy savers.