I spoke to SEIB about the BHS insurance when I was looking at options for horse insurance. From memory, the BHS T&C's were something along the lines of "anyone else in control of the horse on an occasional basis". I checked with SEIB that it would cover someone riding maybe once a month and was advised that this would be considered "regular" and therefore not covered. They also said that the BHS insurance is to be considered a "policy of last resort" and not really something that you should be using to cover yourself for any specific or regular risk.
However I went to look for the current BHS Gold T&Cs and notice that the insurance is now provided by "Markel International Insurance Company Limited" and the T&Cs have therefore changed substantially. The full T&Cs are 54 pages long and not very easy to navigate nor extract definitive answers from. Gotta love insurance products.
The key facts for the current insurance has the following extensions -
1) Authorised Users’ Indemnity.
The Master Personal Liability Insurance extends to indemnify any person to use the
Assured’s horse or horse drawn vehicle or a horse or horse drawn vehicle normally in the
Assured’s custody while using such horse or horse drawn vehicle only subject always to
General Condition (5).
2) Grooms Indemnity
The Master Personal Liability Insurance extends to indemnify any groom while working
the Assured for liability arising out of such work for the Assured.
Provided always that such persons covered under Extensions 1 and 2 shall as though
they were the Assured observe, fulfill and be subject to the Terms, Conditions,
Limitations and Exclusions of the Master Insurance.
I suppose you'd need to read and understand the 54 page policy to know whether the insurance is suitable for your needs (and even then I think they have ways to wriggle out of paying up!).