Choice is yours, however £400 + £100/year (and you'd claim this back as a running cost of the business when you did the accounts) seems a very low price to me for ultimate control over my domains and those of my customers. I'd have thought that a few of your "lost" domains would pay the Nominet bills, however I'm only trying to offer technical advise, not how to run your business.
Good luck.
Good luck.