Simon, if you intend to restore/register the rolling chassis at some point in the future you should get it registered ASAP! It is far easier to get a car registered in the U.K. than almost any other European country if it does not have documents. You should immediately (TODAY!) do a NOVA declaration, this should have been done within two weeks of importing it. You will then be able to go through the registration process. If you don't have documents you will have to get a dating letter from the Club (I think it is £150 regardless of whether you are a club member or not nowadays) and there will also be the DVLA registration fee to pay, and obviously to get it registered it has to be insured.
I think it would be much easier to get the rolling chassis into Sweden if it has documents. From my experience of driving through Europe, if you have all the paperwork you don't have problems. If you don't intend to restore/register the rolling shell, I would take it apart so that you can just take the parts to Sweden, i.e. it is no longer a car.
Don't forget, as of Friday we will no longer be in the E.U.