AFAIK the freeholder cannot refuse to renew the lease without paying compensation.
Whether this is a good deal depends on the open market price of a similar flat - but with a longer lease (over 70 years) and the £50K you have been offered. It needs to be greater than the Marriage Value.
The marriage value is the increase in value, split between the landlord and tenant, using a complicated laid out formula that a flat with, say, a 99 year lease would bring.
You can get an idea of how a marriage value is calculated from http://www.lease-advice.org
In simplistic terms (just for example - the actual calculations are much more complicated), if a property with a 99 year lease is valued at £99K and worth nothing if no years remain, then using straight line devaluation at 13 years remaining it's worth £13K. Take the current value (£13K) from market value (£99K) leaves £86K. The tenant would thus have to pay the landlord half this as the "marriage value" which is £43K.
David
Long ago, below an picture of a 4-masted sailing ship in a heavy storm was added: "A ship in a harbour is safe, but that's not what a ship is designed for"