To merge all of the accounts into one the registrant has to be exactly the same.
If they are only slightly different e.g. "My Company Ltd", "My Company Ltd." then you can go into the relevant account, alter the registrant and specify something like "Adding full stop to consolidate accounts" as the reason and Nominet will process it pretty quickly... but...
Be careful. If Nominet notice that the registrant has been dissolved they may cancel the domains...
It might be safer to just initiate multiple registrant transfers to a new account and pay the fees.