Service always starts from row 1, I've never been on a BA shorthaul flight where that was different. Row 1 will be kept for Golds/Emeralds, it may be released at a late point. Seat prices are at their lowest at the point of initial ticket purchase, they then typically go up if buying later on, they sometimes come down but are not supposed to go below that initial price. The most usual scenario at OLCI that thanks to Theoretical Seating you will be offered a block together, particularly if you are on a reservation with an earlier sector (since you can check in earlier than point to point passengers) but it can't be guaranteed without payment, and it is more likely to be towards the back of the cabin - the curtain will be pushed back in line with CE sales. If it really isn't the end of the world if the teenagers are a row or two away from the parents then I would point out that £20 x 4 buys a lot of pizza.