We did a major renovation with side return extension, total re-jig of upstairs and downstairs layouts etc.
A few bits of advice:
1. We also wanted to do the loft but ditched it due to budget. But, while the house was a mess anyway our builders recommended spending a bit of money on putting the steels in place and the electrics/plumbing in so that when we do eventually save up enough money to do the loft in the future, it will be a lot easier and less disruptive.
2. We employed an architect and wished we hadn't - we could have put the money towards the loft! A good building contractor will retain the services of an architect and can draw up the plans for you if you know what you want, and they will put a project manager in place to manage the build from start to finish. Unless you are after a "Grand Designs" type building, it's really not worth paying an architect.
3. We used PVA Development for our build as they came highly recommended by two friends. They were brilliant and we have since recommended them to several friends and friends of friends who have been equally pleased. If you want to pop round and see ours as an example of their work do PM me.
4. As a few others have said - make as many decisions before the builders start as possible. Make sure you know exactly what light switches, sockets, paints, sanitary goods etc. you want and whether you want door knobs or door levers - all those tiny decisions - in advance and you'll save a lot of stress later and a lot of phone calls every day from the builder asking you to make the decisions quickly as you are holding up the build!
Hope that helps - good luck and enjoy it! The upside of the stress and the cost of a big project is that you'll get a home that is exactly as you want it