You don’t say if you have been given notice to quit so I’ll assume you haven’t.
If the landlord sells and one of the conditions of sale is vacant possession they only have to give you 30 days notice.
If they don’t sell with vacant possession, then the new owner would have to give you 90 days notice (but the chances are that they would want vacant possession if they were going to ask you to move out anyway).
They may, however, want to start you on a completely new lease (perhaps to increase the rent).
All of which comes down to too many imponderables and “handshake agreements”. I would tell the agent that you want a new lease before the sale goes ahead that will take you to, say, one month after you return from overseas.
Failing that, give them notice that you are quitting the flat and arrange to put your stuff in storage (which will be a lot cheaper than renting an empty flat) then start from scratch when you come back.
You can find out more about your rights HERE and if you do put yor stuff in storage, you can still find a new home on the internet HERE