Depends on what Withams let you do. When I bought mine at MVS years ago, they MOT'd it, I arranged the insurance, took the lot along with the MOD cast document to the DVLA office got the reg number, got the plates made, arrived at MVS, they fitted plates and I drove it away. Alternatively, you can, as you say, trailer it away. When I bought mine at MVS, it took about 2 weeks to get to the reg plate stage due to work getting in the way.
Nige