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Hi, this is perfectly possible.
It would be easiest for you to sell the house first, in that all that will need to be done is for you to ask a Surveyor to prepare a Land Registry compliant Plan showing the extent of the house and land to be sold. As and when the Buyer completes, his Solicitor will then register this Plan at the Land Registry together with the Transfer, which will include a provision granting the necessary rights for drainage and any necessary rights of way over the land you retain. The Land Registry will grant a new title number for the house. You will then be left with your existing Title number, on which the 2 cottages are.
This is pretty straight forward procedure for a Conveyancing Solicitor.
I hope this assists and if so, I would be grateful if you could rate my answer in the top right hand corner of your screen.
Kind Regards Al