As far as I can see, this is no different to an involuntary downgrade at the airport. In that scenario, they're required to honour the 2pc baggage allowance. Just front up with the ticketed allowance and let them sort it out.
This is also relevant (Source CAA)
Under EU law, you have certain rights if you are downgraded. To be covered by these rules, your flight must be either:
- departing from an EU airport and operated by any airline
or
- arriving at an EU airport and operated by an EU airline
If you are downgraded, your airline must reimburse you within seven days. The amount you receive is calculated as a percentage of what you paid for your ticket, and depends on the length of your flight:
- For short-haul flights of less than 1,500km, you will receive 30% of the price of the flight.
- For medium-haul flights of 1,500km – 3,500km, or flights within the EU of more than 1,500km, you will receive 50% of the price of the flight.
- For long haul flights of more than 3,500km, you will receive 75% of the price of the flight.
- It is likely you will only receive a refund for the portion of your journey that was downgraded.For instance, if you booked a return ticket for £1,000, but were only downgraded on the return leg, your reimbursement may be calculated as a percentage of £500.
So it's 75% of half the ticket price for the BOS-LON leg