Me and two family members are considering going to Stockholm with BA for a weekend away using a mix of tesco vouchers and my LAN miles (will get on to them later). However I have few questions I need clearing up.

My mum has about £150 worth of tesco vouchers that she'll use to convert to BA miles into her Exec club and pay for her and my sister.
-Am I right in thinking that BA miles only cover the cost of the flight and taxes and charges will be extra on top?
-Can you choose any flight to go on or is there certain availability per flight for BA miles customers?
-Can she use her BA miles to purchase tickets for her and another passenger?

I'll be booking the flight using my 40,000 miles from my LAN pass account. Is it the same deal here or is there anything I need to be aware of?