It is possible to do this.
You simply you form TP1. This is a standard transfer deed used when only part of a registered title is being sold.
You will need an accurate plan identifying both the land being sold and the retained land.
The next part may be complicated, depending on what is being sold and what is being retained, you need to consider if any rights need to be granted and reserved (i.e rights of passage of services, rights of access, rights to enter to carry out repairs, rights of light to be reserved if you have plans to develop the retained land etc) and any covenants (Legal term for obligations, i.e restricting the use of the land sold and obligating the buyer to contribute to maintenence and repair of any shared access and dealing with who has responsibility for which boundaries). Your plan should mark who will maintain and repair the boundary features (shown by a T mark).
This, of course, is very general. The transfer would need careful consideration and should be drawn up by a solicitor. No doubt the transaction should follow the standard Conveyancing procedure, your solicitor deduces title and drafts the contract and transfer. You will have to respond to enquiries raised.
Not knowing the property, not seen title etc, I cannot advise on whether a lease is a viable alternative. You'd need valuation advice on freehold and leasehold. As to the term, 75 years is inadequate. No lender will lend on a 75 year lease. May not be an issue if buyer is paying cash but could be an issue on resale. After two years a tenant can apply for a 90 year extention at a £0 ground rent.
Which ever way you proceed you need tax advice, solicitors are not qualified to provide this.
Hope this helps.
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