you could build under "LAWFULL DEVELOPMENT" as long as the following is true:
1) the "SHED" is no higher than 4m if pitched roofed or 3m if flat roofed;
2) the existing house has never been extended before
3) and the "SHED" is a temporary structure at least 1m from all boundaries
4) and the "SHED" does not take up more than 50% of the garden or 50m^3 using external dimensions.
If a full planning application where submitted then the "SHED/WORKSHOP" could legally be called a "HOME CINEMA"; but this would need to comply with buildign regs as a habitable room and insulation and sound proofing would also need to comply.
However, you should get in touch with you local planning office as each one has some slight variations to the lawfull development requirements due to local bye laws. They will also be able to put you in touch with a local "Planning Consultant" if you want to proceed with an application for a "home cinema in a shed".
Good Luck