You can't merge bookings. If you were Gold, you could link (TCP) the bookings and select seats across both. The only way to ensure you sit together is to pay for your parents seats in advance of online checkin.
If your parents were nervous and needed to sit beside you, then they'd need to decide if £88 was worth the risk of not getting seats together at checkin.
And I'm afraid you did sound a bit DYKWIA. A Silver travelling with two non-status passengers are inconsequential to BA. Don't fret though, individually, most Golds are inconsequential to BA too.
Last edited by Paralytic; Jan 23, 2015 at 8:50 am