You are not obliged to allow viewings if you don’t want to. This is regardless of the contract. Lots of contractual terms are unenforceable.
I would contact the Landlord direct for clarity and ask for open and clear communication through the process. I would point out that notice has not been served and you will be expecting the right approaches to be taken to this. I would question whether he is looking to sell to a LL and how explicit this requirement .
In reality there is likely to be a be a void period. LLs have to accept when selling that unless they sell to a Tenant in situ, the place will have vacant possession. Most realise this needs to happen before marketing, but some are greedy and hope to keep a tenant until…..well quite who knows what their thinking is or how it fits with giving notice and then letting people move out when ready.
I bought a property with a tenant in situ.The details themselves didn’t mention it, but I was called about the property By the EA as was an investor on the agents books. They called me and told me it had a tenant who was interested in staying. I viewed the property and wa sooner minded about the tenant. I discussed them with the letting agent (also selling) and it turned out they had been there 2 years and always paid their rent. This was good for me and they stayed on for another 3 years.
The seller though was facing difficulties. They were in mortgage arrears and under pressure to sell or face repossession. I was a cash buyer and didn’t need a mortgage so issues of having a tenant in situ was less. As a cash buyer with no mortgage or chain I was able to get a good price.
Whether your LL wants to sell to another LL depends on an variety of factors. Sometimes this can be good and straightforward, but sometimes it can result in a lower price. To be honest, most LLs only sell to an investor with the tenant in situ if this is to their own advantage.
I would be expecting to need to move. I would start looking now so you can go on your own timescales and not theirs. You will then be more in control of what happens.