I can't see EC261 / APR working here, it's really not the point of that legislation. On the other hand the Consumer Rights Act works on a different basis, namely you not getting what you originally paid for. Now there is reasonableness aspect to that, so if you want (e.g.) a window seat and you can't get a window seat then you seem to me to have an arguable case, based on no additional spend. If you end up paying money for peace of mind, well that's more nebulous since you would have knowingly entered into that, and you would be getting something for the extra cost, so ironically my perception is that won't be refunded if taken to small claims. Having said that, we all know that for all airlines getting a particular seat is up against carefully constructed conditions of carriage which in essence protect airlines to the nth degree allowable.

In the case of Finnair, the number of people able to claim free seating is somewhat diminished, so that should mean that you will be OK at OLCI, most passengers simply are not that bothered about it. So personally I would not pay extra and then, if the travelled seat is less than ideal, as an option complain to BAC afterwards about the result. As already indicated you may well earn on this flight, and if that plus any BAC Avios gesture gets you over a fairly low threshold of 6,000 Avios then overall you wouldn't be doing too badly.