as others have said.
1. get a decent accountant (if you haven't already) and ask them
2. you cant really split them. you'll be fine doing so .. as long as you don't get looked into. you might get lucky, but not worth the stress.
3. if you want to grow get VAT registered and make the savings where you can. if you split them you have 2 sets of accounts, 2 mail boxes, separate insurance, separate fuel cards etc etc. lots of your time wasted
i went VAT registered about 5 years ago. it was a massive strain for me agonizing over the decision you're making now. make sure you have your accounts and business running smoothly before you do, and make sure you put your prices up (i wouldn't recommend 20% though, i did £1 per domestic client which brought in about £450 extra a month)
now my accounts are running very smoothly and my business is simpler as a result. i pay VAT monthly myself so i dont get behind
R