Hi, I have done the same thing about registering an import, there are some pitfalls which can be very expensive so be careful.
Step 1- Complete a SORN with HMRC and do it within 14 days or the fine is £5 a day after this.
Step 2- Get a vehicle import pack from DVLA by phoning them
Step 3- Get an MOT, just a normal one. (They use the VIN to register it).
Make sure you bring the Polish registration document (V5 equivalent) as they will need it to register for MOT. Tip- you will also have to swap the place of the fog light and revese light onto the opposite side and maybe get RHD headlights depending on the tester. Don't worry about the speedo being in kmh as this wasn't a problem for me
Step 4- Buy some insurance (insured on VIN). Make sure this is done after the MOT because some insurers will require a reg number after 14 days, so if the MOT fails then you may not have enough time. I would recommend Adrian Flux as they are a lot cheaper for imports for me.
Step 5- Send all of the requested documents, MOT certificate, Copy of Driving licence including counterpart, Polish V5 equivalent, insurance. A cheque for £55 (registration fee) and a cheque for £145 (tax) and also the form which requests details of the vehicle (weight etc...). TIP, get a certified copy of the registration document in case the documents get lost in the post as without it you will have a nightmare.
If you need any help dont hesitate to ask me.
