I don’t think asking for a premium to take on a lease in FH will be easy, so this may well be as @Anotherjohn said a very long process.
As far as I know accounts will probably have to be shown to justify a premium and most would probably expect a 5 year history, so even more difficult for a newish business.
I know of a few businesses in FH that wanted a premium for their lease but ended up passing on the lease without the premium to move on.
Whatever the case it probably won’t be easy.
I don’t know the situation or the reasons why but If the owners really want/need to move on I would say the best bet would be to speak to the landlord and ask if they can leave the contract early if that is what they need to do. Depending on the clauses they may or may not be able to do this.
We don’t know what the rent and overheads are before a profit is made, so can’t really judge on this without knowing.
I only know a bit about this as I almost took on a second shop premises in Hoxton a couple of years ago &'when others started showing an interest after my initial interest (which had no premium) the landlord added a premium last minute which put me off straight away so it made me very wary.
Sorry boss has to be said here @Anotherjohn is a local shop landlord and you couldn’t find a better and fairer shop landlord than him in FH!
His advice on this thread is probably invaluable