Solutions in order of easiness
1. Agree to buy car, go home sort out insurance, come back a few hours/days later, drive away. Deposit if necessary
2. Ask the current owner to drive it over (presuming they have it insured) and then get a friend/parent to drop them back at home, give them a few extra pounds to do this as long as its not too far
3. Get in touch with a car transporter company, cost me ~£30 to move it on the back of 7 1/2 tonne cab about 10miles to a garage
4. Get a parent/friend with the drive any car insurance part (carefully check this and that the current owner still has valid insurance) and get them to drive it for you.