I looked into this prior to getting my 100HP and the DVLA have plenty of 100HP related plates to choose from for £250 all in, including VAT & transfer fee.
You'd probably recoup your money at trade-in time at that price - if you leave the plate on the car.
As long as you don't mind messing about with the spacing - and yes, before you all start - I know it's illegal but the Feds would be better spending their time nicking the mobile phone users and other reckless drivers.
eg:
P10 OHP (P100 HP or P 100HP)
P100 BHP
There were others but I don't have the list I made to hand.
Ultimately, unlike my ALFA (H** GTV) I decided I didn't need another 100HP badge on the car. But it was fun looking. Hmmmm....must get out more.