Frames

There's no single best frame, it depends on what you're building for. Freestyle frames prioritise durability and camera protection, racing frames prioritise stiffness and low weight, and cinematic or long-range frames prioritise space for bigger batteries and HD video gear.

Building from a frame up is generally cheaper than buying a ready-to-fly drone at the same spec, since you're not paying for someone else's assembly labour, and it lets you replace only the part that breaks rather than the whole aircraft.

Yes, with rules attached. You need an Operator ID from the CAA (£11.13/year) for any drone over 250g or fitted with a camera, and a Flyer ID (a free online theory test, unlimited attempts, valid five years) to fly it. FPV flying specifically also requires a competent observer maintaining direct visual contact with the aircraft and able to communicate with you, unless you're flying at a BMFA-affiliated club site with an exemption, or covered under an FPV UK membership that permits solo flight under a site risk assessment. Standard limits still apply on top of that: 120m maximum altitude, no flying within airport Flight Restriction Zones, and 150m from buildings in the open A3 subcategory.