It is an offence under British law to connect non-compliant equipment into the UK phone network...oh boy.
That said many people do, and you'll only get caught if the equipment in question is causing problems for BT at the exchange (central office).
All telecommunications equipment in the UK has be be compliant with directives from BABT:
From Wikipaedia:
The British Approvals Board for Telecommunications (BABT) was established in 1982 by the UK government to provide type approval services to the telecommunications terminal industry.
You can contact them at the following address:
customer.services@babt.com
Depending upon how much you want to know also try reading:
61 pages though.
UK compliant kit used to have a green circle sticker on the underside (in the UK).
Problems could arise if the impedance of the handsets you mention does not provide the correct loop impedance required to flag the exchange.
Tolerance to ringing voltage (88VAC upper 100v/ lower 66v) and any reflectance (impedance mismatch) generated by the handsets may be a bigger issue for your brother and/or BT.
Most UK handsets (domestic) are Time Break Recall (TBR), with MD (DTMF). In addition the handsets may benefit from the ability to operate in Earth Recall mode and Pulse Dial.
On the other hand, handsets are available for less than £10.00 from certain well known stores.