Hi I recommend Aura Homes who specialise in design-led high-end extensions. They are based in Clapham . Aura Homes would discuss your project further and guide you through the design and construction stages. They have a simple 10-step Home Design Guide they can share with you to explain the process.
I can recommend Toynbee Associates. They did structural calculations for two internal wall removals in our renovation project. Tristan Toynbee (the founder) was very competent when I spoke to him numerous times throughout the project. However, I did struggle with a more junior person in his team that was originally assigned, and at times it felt they had lots of work and so there were some delays. The end result was good though, and the pricing was competitive.
I would at all costs avoid Abbey Partners. I had initially used them, but their cost was astronomical, and after payment and the delivery of the first set of calculations (which were handwritten and hand calculated no less) I found them to be rude, slow, and uninterested in follow ups to their original calculations.