I see a number of pitfalls:
Assuming you do pay your Dad enough to purchase your third (say £100,000) before he passes on, how do you propose to then buy out your brother and sister when they inherit the other two thirds? You will need about £200,000 (at today’s prices) to remain in the house, otherwise it will have to be sold and the proceeds split three ways.
To pay your father £100,000 in (say) ten years you will have to pay him £200 per week.
He may encounter tax problems on this additional income of £10k per annum.
There are a number of other problems which I think would be best addressed in your visit to a solicitor.