The MCA have agreed that candidates applying for an Unlimited CoC may use service gained on Yachts >24m.
This includes OOW, Chief Mate and Master Unlimited, however you must gain the OOW Unlimited before being able to progress through the Unlimited CoCs.
Many of the courses required are the same ones you have already done for your Yacht CoC - all of the Basic and Advanced safety courses and GMDSS will not need to be repeated (see current regulations regarding revalidation and updating training).
The specific training required for the Unlimited CoCs needs to be completed at a recognised College, however some training establishments offer 'blended learning' which involves some distance learning and some college time. This enables you to continue working at sea while studying towards the Unlimited CoC