Thanks for the replies I've been advised by the solicitor handling the purchase that we will have to pay the additional stamp duty on our future main residence as we won't be selling the property that we're buying first... if that makes sense
It appears to me that you might want to seek advice from a solicitor with a more holistic interpretation of the rules ...
The simple interpretation is that you're selling your main residence, buying a new residence (the future holiday let) not subject to the 3% uplift, then buying a further main residence which is subject to the uplift because you're not disposing of your current property.
However, HMRC guidance (
https://www.gov.uk/hmrc-internal-manuals/stamp-duty-land-tax-manual/sdltm09812) is explicit that "Merely occupying a property will not in itself make it a main residence. There needs to be a permanence and expectation of continuation to the occupation to establish it as a main residence."
Further, "The test in respect of the new dwelling purchased is a question of intention, does the purchaser intend the dwelling to be his only or main residence? This is a question of intention at the time of purchase. What has the purchaser acquired the property for? [...] If the dwelling is intended to be put to other uses, for example as a source of income, then the intention test will not be met."
Your situation seems similar to case study 7 in
https://www.blakemorgan.co.uk/wp-content/uploads/Downloads/SDLT-case-studies-for-3-surcharge-20.01.20.pdf - it might be possible to sell your current main home and buy the future holiday let without uplift (because you don't own a second property), occupy the FHL without it becoming your main residence (because you don't intend to be there for any length of time), then buy your real next main home without uplift (because you had previously disposed of your current main home).
You'll probably want a sympathetic solicitor to let you run that one though - and it might be easier to demonstrate that the future holiday let *isn't* your main home if you instead move into a caravan or a short term rental ...
(I think this guy might be a suitably sympathetic solicitor ...
https://www.blakemorgan.co.uk/people/john-shallcross/ - he does Q&A on the Zoopla blog at
https://www.zoopla.co.uk/discover/buying/q-a-new-3-stamp-duty-surcharges/ as well as the case studies linked above)