Section 15 Seat Strength - details the testing that should be carried out and the requirements of the installation.
Head rests are a further issue you need to address with the seat, but the main issue is where the seatbelt is anchored or if the floor is metallic or not.
This is a copy of the 24th April 2009 dated issue (referred to as the May 2009 version/release 3) for reference:-
http://www.transportoffice.gov.uk/cr...20May%2009.pdf
Read it and make of it what you must.
Use the testing proposals to judge your interpretation.
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