I think I know the answer already!

Well, probably not
The rules are pretty clear. '...and Business Advisors' is allowed, but not Planners.

However, the rules don't prevent you omitting the 'and'. So your letterhead might look like this

XYZ & Partners
Chartered Accountants
Business Planners

I personally think the restrictions are pretty daft. If all they wanted to do is prevent firms using inapropriate descriptions, that's already covered in the rules.

Not really relevant ...
but my favourite is a firm that advertise themselves as "Licensed Trade Accountants". Now the clever bit is that they are not at all licensed by anyone but the do have rather a lot of pubs as clients! Brilliant.