Is international mail leaving England and Wales only being sent by boat and not by plane due to the virus?

Most International mail, goes by plane as it's faster.

In the 7 months or so of the main Covid emergency, I have not heard of any restrictions on airmail.  I live in Britain, and have relatives in Canada, and my wife was from Australia, so I have an interest in such matters.

Of curse, there may be some delays of a small number of days because fewer flights care happening, but there id no prohibition on air mail.

Who or what gave you that idea?

Note that mail to Scotland goes by train.

No, it's not boat only.

Planes are still going everywhere - but often now carrying other things (like PPE) rather than passengers. 

So airmail is still going by plane - but might be delays given less planes are flying generally. 

No, freight and mail flights continue as before

There is special planes that carry mail and parcels etc

No , 

planes are still flying over my house.