All of Rod's advice is correct. In reality you cannot build one without LAA involvement. ( In theory you could build under the CAA and operate on a CAA permit but this is not a practical route).
You cannot have the aircraft built for you as it invalidates the LAA system ,however you can receive an enourmous amount of help.
Vans aircraft have the best support of any kit aircraft within the UK and there is probably more experience and back up for them than any other type because of the vast number built in this country.
Vans owners/builders have a website which you will find helpfull .google Vans uk.
First port of call is the LAA.
Make no bones about it, it is a huge commitment ,you are about to embark on a journey that will take over your life for several tears ( that was a typo ,but may be accurate!) years.
If thats not for you ,then think about maybe importing one,
www.barnstormers.com is a good site,but they go for good money and it may be an option to start a group.