I'm sorry to hear of the bad surprise you got! Because your flight was cancelled and flights are very full this week, I imagine that BA did not have enough spare J seats to rebook everyone in the travel class they were booked on. I imagine that this is why you ended up getting the short end of the stick and they put you on the first available flight they had seats on albeit in the wrong class. If you prefer to wait longer but fly in J as booked (I would prefer that in your case if timing was no issue), I would certainly tell the sales desk at the airport and ask them when you could fly in the cabin you actually booked. You can then decide if the additional wait would be acceptable or not, as it might be different for you if you end up having to waste 5 more days or just a few hours.

Depending on the reason for the cancellation, note that you may be eligible to compensation for the cancellation of your original flight under regulation EC 261/2004 (look it up on this forum if you are not familiar with the details - lots of threads). Also EVEN if the cancellation was due to extraordinary circumstances (such as a weather issue), if you are flown in a lower class than booked you are eligible to a refund of 75% of your fare under the same regulation. If it changes your mind on whether this would then be acceptable or not I would let them know as well.