I am assuming that they are not currently in a fixed term lease.
Therefore, under Part 4 rules you are entitled to evict the tenants on the grounds that you intend to sell the property. Your notice period is 84 days as you know.
However, from a tenant's point of view, I would not leave finding a new place to live until the last moment. Knowing that I had to move, I would start looking and once I have a property I would vacate. I doubt very much if a tenant would stay the full 84 days.
IMHO, you should just have a talk with them and agree to a shorter notice period between yourselves. A shorter notice period if agreed mutually between the parties immediately prior to or after a Notice of Termination is served, is perfectly legal. It is illegal to put a shorter notice period into any lease or other agreement.
Thus, the tenant will probably leave without the necessity of a cash incentive. However, if they were good tenants, you might like to lighten the blow of an eviction with some cash incentive for a sooner rather than later exit by the tenants.