You would be entitled to reimbursement under the Mennens formula - for a long-haul flight such as this, you get 75% of the fare plus carrier surcharges back on a pro-rata basis (based on the percentage of the overall trip distance that the affected leg makes up).

However, it might not be entirely straightforward getting that reimbursement, seeing as:

1. You've booked a Holiday rather than just a flight. Any effective discount by combining the flight with the other elements might be deducted from the fare, although to be fair, you are quoted an all-inclusive price when booking, so that is probably not entirely unreasonable.
2. BA might try to refuse reimbursement on the claimed basis that you have "accepted" this change, or that they provided more than 14 days' notice. Neither are valid grounds for refusing reimbursement, but that doesn't stop them trying it on.
3. BA might try to exclude the carrier surcharges from the amount reimbursed. This often makes up a significant proportion of the total fare, albeit less so for most F fares. As long as the carrier surcharge depends on the class of travel (it obviously does in this case), a recent CJEU case made it absolutely clear that it must be considered when calculating the applicable reimbursement.

For now, I'd advise getting a PDF showing the details of your fare and booking from the Finnair site (it will email it to the address registered on the booking). This clearly shows the breakdown of fare and carrier surcharges so that you know whether BA have got their calculation right, if it comes to it.