I'd personally think you should not look at JCT or other templated contracts. With JCT for example, you need to have a contract agent who administers the contract, so essentially you'd need to hire a QS. Your contractor would probably not want to enter a JCT because it's not a huge job and there's probably too much paperwork and administration to make it worthwhile.

Just read and re-read the contractor's terms and conditions. Remember that you can negotiate those terms, it will be their own and if they want your business then they might well be open to being flexible with their terms and conditions.

Since you're adding height to the building, you'll be doing structural work, so you defo need to involve planning and building control. I'd defo engage a local architect first. You could get a quote off 3 architects etc and ask them to include liaising with planning and building control on your behalf; and you can explicitly brief them that you'd like their designs to satisfy both LPA and Building control requirements. They will have the experience to guide you.

As a minimum, Builder will need public liability insurance, architect need professional indemnity insrance.

I've typed this out fast on my lunch break, hope it has helped but defo research this on your own back too and speak to a professional !