To be honest, you can probably claim it in either tax year - as long as the result doesn't distort your tax position.
Strictly speaking if you do returns on a cash basis, which you're not meant to but most landlords do, it's last year, on an accruals basis it should be amortised, but is more likely this tax year.
The prevailing view is that expenses incurred against aborted purchases are not allowed, and there isn't really anything that counters that.
If you genuinely hold the view that an expense wasn't actually tied to the abortive purchase and was an expense solely and exclusively for the business you can claim on that basis.
When I post, I am expressing an opinion - feel free to disagree, I have been wrong before.
Please don't act on my suggestions without checking with a grown-up (ideally some kind of expert).