GoSeigen wrote:I think this is poor advice.
Very likely, but ...
There is absolutely no need to pursue this path, because your landlord cannot take possession without returning your deposit.
That is surely a red herring. The landlord apparently[1] can't serve notice without either protecting or returning the deposit. But that's not the issue here: the tenant intends to leave anyway. He gains nothing by continuing to possess the place after he's moved out! If the S21 notice is defective, all the tenant gains through that is more time. Which is indeed a Good Thing to have, but that's another question.
I'd just wait for the appropriate time to put it to him that you are owed your deposit, and that as a condition of your agreeing to vacate "early" you want an undertaking from him that he will repay it in full. If he is nervous you could allow him to inspect the premises in advance, and undertake to leave it in substantially the same condition when you actually vacate .
The risk there is that he has to go to court for it, which is a lot of hassle he could do without[2]. If he continues to possess the place against the landlord (which is the legal weapon he probably has), then more rent is due for that time, which is the last thing he wants! Plus of course he opens himself to court proceedings.
Withholding rent for any reason is not only rude but is a serious breach of your contract and may open you up to a section 7/8 proceeding. Your rent is payable in advance and so I see no scope for substituting rent for deposit.
GS
Turfing a tenant out of his long-term home is a whole lot ruder and nastier! And this is a tenant in good standing, who has a demonstrated track record of cooperating when he could have taken a stand over viewings.
My suggestion was - in effect - that any section 7/8 proceeding will be overtaken by events, as he intends to move out anyway before he can be evicted. Doesn't the rent have to be nearly a month overdue before a landlord can even start such proceedings? So in practice any proceedings could only seek to reclaim money, and that money will be offset against the deposit. Thus the only real risk is that he's then likely to be in a weaker position if he needs more time than he thinks to find a new place.
[1] I really wouldn't be comfortable relying on that S21 being invalid based on anything posted here, even when apparently backed by a reading of the legislation.
[2] And all it gets him is a court judgement, which the landlord might then ignore. Especially if the landlord is hard to chase (e.g abroad).