I can probably help here, as I was asking the same question last year.
It turns out that yes, you can have an accessory only order, provided you have had a bike before. I researched this is some detail last year to ensure I wasn’t doing anything “wrong”.
There is a list of what is allowed & not, trailers didn’t appear on either list – they were apparently on the “not allowed” list a few years ago, but not any more. The rules are set by HMRC, and then interpreted via the scheme you’re using, company etc.
I now have a Thule (Chariot) Cougar through the scheme, a trailer enables dropping the kids off at nursery, it’s accounted for properly on the application form, quote & reciept as a trailer, so I’m confortable it’s all official.
When researching, one shop (no, I’m not going to say which one) did offer to put a bike on the quote, but actually provide a trailer, but I didn’t want to go that route.