You might want to look for a car power inverter. The link here is for one with a US outlet as an example. It can handle 150 W. It can also most likely be used with a UK outlet converter.
Longer answer: The SP3 power adapter provides an output of 12V and 2.58A DC. That is the max power output from the power adapter to the SP3. Power is calculated by 12 V x 2.58 A = 30.96 W. I would round up to 31W.
In comparison, the current iPad is 12 W, and the current iPhone is 5 W. If you want to be able to power devices simultaneously, then you'll want to find out how much power other devices you would like to plug in would take. You might also want to add a safety factor ;-) For an SP3 and two phones, 50 W should be sufficient.
For myself, I would probably check how much power our laptop takes, and I probably would end up getting a 100W or 150W power inverter with multiple outlets. However, if I was getting one just for my SP3 with one outlet, I would probably go with 36+ W and compare prices. Actually, I would probably try the one I included the link to since I could plug my phone into it by USB, doesn't cost a lot, and has good reviews.
It is odd... my buddy who is a SP3 i7 owner just said the same thing to me this morning... "when is MS going to offer the car charger??"
I told him the same thing, get an inverter. But he said no way, that MS makes one for the SP2, which he had, and loved it because it was small and portable and knew he could throw it in his bag and top off between meetings or whatever. No way he could do that with an inverter, driving different company cars etc.
I did not know that MS even made one in the past.
So now I have different insight into the car charger issue.