I have been given a section 21 [of the Housing Act 1988] notice but I don't have an address in England and Wales to which I can service documents to, as specified by section 48 [of the Landlord and Tenant Act 1987]. Does this invalidate the section 21?
According to an article on Landlord Law Blog which addresses this very question, no:
Section 21 notices must comply with the requirements of section 21 of the Housing Act 1988 and the courts are very strict on this. However, I cannot see anywhere in the section a requirement that the notice must give an address for the landlord which is in England or Wales.
The article also confirms one aspect of the question, in that:
There are situations where the landlord’s address is significant. For example rent is not due unless a landlord gives an address for service of documents in England / Wales.
The question also states:
If I want to demand to know the address of my landlord, I can [write] to him abroad, but I won't be able to claim proof of receipt for letters abroad... and sending recorded delivery abroad is [expensive] so would rather not do that. So how can I reliably ask for his address?
I'm taking this (and subsequent comments) to mean that OP does have a reliable way of contacting the landlord - but that the landlord is in breach of section 48 because no address has been made available which is in England or Wales.
So the answer seems to be: assuming the Section 21 notice is otherwise valid, the fact that the landlord has provided an invalid address doesn't change that. However, it appears that no rent is due until a valid address is provided (though note that once a valid address is provided, all withheld rent is then due.)
It is possible that the landlord has made an honest mistake, and didn't realise that the current absence of a letting agent has left them in breach of the law. So it may be worth contacting the landlord to point this out, so they can appoint a new letting agent.
Please note: withholding rent when it is legally due can have serious consequences. I am not a legal professional, so before taking any action, I would seriously recommend talking to an expert.