Yes BTS is operated by a based BRS aircraft and shall be parked overnight, as long as it isnt stranded somewhere in Europe. With BRS being a 5 A/C base they should have 1 spare A/C but I know BRS is stand restricted so this might not be the case.
In terms of your BTS - STN sector this is classed as a separate booking and therefore would not be refundable to you.
Hope that helps and fingers crossed!