Hi, I would start to think about the costs in four buckets: Professional Services (Architects, Party Wall Surveyors, Structural Engineer, etc), Prime Building Contractor, Kitchen/Cabinetry and then Fixtures and Fittings.
As a very broad rule of thumb:
Professional Services: 15%-20% of total project costs
Build costs: £90-£150 per Sqr Ft
Fixtures and Fittings: It really does depend on the volume/quality
Hope this helps, call if you'd like to discuss, all the best, Mike Lander, Director, www.ensoul.co.uk
020 3637 0700