His contracts for the two "separate" services end anyway on the anniversary of his last renewal of each. Prices revert to the standard figures that you can find online in
the Legal section at the bottom of their website pages.
I haven't gone into it in depth but there used to be and maybe still are different Terms and prices depending on when he joined in the first place.
That continuation will be on a monthly contract basis, nothing to stop him migrating away at any time afterwards.
So you/he have a choice between continuing at the default price applicable while you decide whether or not to move; sorting out a migration rapidly to take effect soon after the move (Ofcom has a specified minimum 10 working days for a migration to take effect, and you mustn't order from the new supplier without ensuring it can't happen while still in contract) ; moving just the phone line to another ISP like I did when with them, (pulse8broadband was the one I went to) and keeping the broadband with Plusnet;
or the probable best way is to
ring and request to speak to the Retentions Team. That last option being what most people who are happy with the service itself tend to do.
Don't necessarily accept the first offer. You can argue and/or ring the next day and speak to a different person and get a better offer.
As has been posted earlier, someone has a very good deal. The expert used to be adslmax, who year after year got incredibly low deals.
Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.
===========================================================================
The price of liberty, and even of common humanity, is eternal vigilance. (Aldous Huxley version of the well-known saying)
Were there none who were discontented with what they have, the world would never reach anything better. Florence Nightingale (Cassandra: an Essay (1860 edition?)