Mr. niknak.
I feel that you may have made an assumption here.
Who said anything about being employed as an ATCO elsewhere?
What if one was employed in a different discipline eg. gardener, plumber, newsagent or flying instructor? Why should nats have the temerity to dictate how one should manage one's life? It is not SRG ATSD who restrict validations at different units. History has proven that. It is not unheard of that Heathrow ATCOs themselves have held validations elsewhere and managed to comply with the requirements which you quote.
No, it seems that nats as a company cannot accept being the underdog and that, perish the thought, nats itself would be in a position as the secondary employer. There are experienced ATCOs out there who would be prepared to provide their services to help get this London LARS off the ground but not at the expense of a lifestyle change just to suit nats' intransigient attitude.