If you're regularly going into London then just look for a car that's guaranteed to be accepted. Otherwise you've got to deal with a lot of faf. Don't have a personalised plate either as they seem to sting You if you have one.
If this is bought from a dealer get it back to them asap as they have to repair it FOC in the first 30 days after sale. And you're entitled to a full refund.
Hi Dan the cheapest option is to have a remap that ksps out the adblue system. That way you won't have a problem with the system again. The remap should cost between £200-500