Depends.
Is the offer they have made conditional or unconditional ?
If unconditional, then outside very specific circumstances, including extreme underperformance (2:2 instead of 2:1+ wouldn’t really apply as that) then the offer would remain. Yes, unconditional does come with some conditions (although they are quite extreme/ unlikely to happen but not impossible)
If conditional, then double check what those are. If contingent on achieving 2:1, then they are within their rights to rescind the offer if you do not get the grade. However, they might not take it back, it is at their discretion.
Unlike undergrad, you are making your application directly to the uni with no middle man. They might still like your motivations etc from your personal statement, and so still want to take you in. You might get a high 2:2 and they still would be happy to have you. They could have space and so could take you to make up the numbers. You wouldn’t hear the reason, so wor th saying not to worrying about the why they take you in (if they do) and just accept the decision.
If you already study there, they usually look on you more favourably too.
You won’t know until the time comes. If concerned, you can always check other universities that might offer similar courses you might be interested in, and apply to them instead as alternatives.
You could also take and break, work etc, and then reapply another time I.e. for next year.
In short, give it your all with what’s left, and aim high and then take it from there.