Im sure they re-issue you with the original plate that was on the car.
They do. I have bought two cars in the last year, both of which had private plates on with the previous owner. In both cases the DVLA simply re-assigned the original number. In one case i then transferred my own private plate to the car. I am quite sure that if I sell it they will re-assign the original number. Your own transfer docs and MOTwill show exactly what happened regarding the original number and the lexus Garage will have a record of the services in what ever number was on it at the time. Don't waste your money on a retention unless the original number has itself some intrinsic value that you can sell.