Who should I use

Depends on exactly what you want done, while a builder will have contacts for varies trades your subject to there tradesmen rather than your individual choices & will pay a premium for them taking on & project managing the job. Good luck Alex

As other said it would possibly save you money with individual contractors but the biggest question is timescale and if you have time to manage projects yourself.

Yes, use a single building firm, but specifically look for a smaller local company rather than a large corporation. Managing multiple individual contractors yourself requires significant construction knowledge, daily scheduling, and endless stress. If one trade falls behind, it delays everyone else, and individual contractors will often blame each other for mistakes. A building firm provides a single point of contact, manages the entire timeline, handles the sequencing of trades (like plumbing, electrics, and plastering), and carries the overall liability for the project. However, you should avoid big national firms, as they have massive overheads and will charge you a premium. A smaller, independent local building firm will give you the exact same benefits of a fully managed project, but it will cost you much less.