If it were me I would decide which type of cycling I like most (on off road) and get the right bike straight away...so if you are set on road riding get yourself a good road bike not a mountain bike...simples but it works out cheaper in the long run to get the right 'quality kit' if you can afford it.
Then just start spending more time on the bike than in the truck.
Don't have high expectations to start with...your body will go through a gradual change and the more you ride the more it will adjust to the demands you make of it.
Learn to ride defensively and assertively (not aggressively...assertively) on uk roads. 90% of drivers are decent peeps, but it's the 10% you need to be aware of. You may find riding with a more experienced cyclist to start with will help with this.
One thing I learned when I went through this..(I lost 8 stone and went from being unable to ride around my local park to riding across America) ...simply by getting out there and riding you are winning, making a difference and will succeed, and have fun. All the pain is worth it!!
I found that just riding with no real purpose other than to get fit didn't work well...I needed destinations. So I started with the park, then the supermarket, then the next town etc..in short I started replacing local car trips with the bike...that worked for me.
When you ride further, remember to eat and drink well. Beware of the bonk!
Get the right clothing, it will make life more comfortable.
Good luck!