is it worth writing to HMRC?
No, as like I mentioned you could have completed a form until last year
how to get into partnership? we are effectively running the properties jointly. can this be classifies as partnership?
Possibly but see definition " Partnership is the relation which subsists between persons carrying on a business in common with a view of profit"
Though I have seen other accountants claim it needs to be formal setup
what is "transfer of beneficial interest"?
The beneficial interest is an interest in the economic benefit of a property. In this case you would transfer say 99% of the property to your wife through a declaration of Trust. This may not work depending if you can transfer the mortgage which the lender may not agree. If they agree SDLT may apply
is there any firm who can do an initial assessment whether it can be done or not?
Most accountancy firms will say yes it can be done as they want your business. Admittedly its such a grey area when transferring properties in a limited company especially when we discuss incorporation relief and remortgaging. Though if it is possible even accounting for the initial fees the savings can be significant due to for lower tax rates (will fall to 17% in 2020) and the fact you are paying 40% on rental profits and mortgage tax relief restrictions. The key is getting the incorporation relief but still unsure you each separately could justify it as a "business" under the Ramsay case. https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg65715
While not as beneficial as incorporating a company. Possibly you could also consider forming a company owned by your wife who would invoice you for managing your properties based on a % of the Rental fee.