as above, you don't have to be Part P registered, you can notify building control direct. Fees vary from £50 to firk knows! The registration is only so you can notify via NICEICs online portal to building control, thereby saving time, hassle and money. If you do more than few domestic jobs a year it's worth the extra money, but from memory AC would give DI be default - I'm not sure about that because I was DI and converted to AC and registration for part P came with it. But I would seriously doubt NICEIC would quibble adding DI status to your current registration with them. Personally I think they should stop having multiple standards and register people as quailifed or not!
Spot ights and sound systems, sounds like interesting work.