Ah my area of expertise!
BG are generally seen as okay for customer service but things always fall through the cracks. I'd imagine there is something in the industry data for your account that needs cleaning up. Are you using a standard UK bank? Just thinking if you're living in Spain there maybe something strange with the DD request (not my area of expertise though).
If you just ring up the standard number its pot luck whether you get a CSA who knows what they are doing. Follow the official complaints procedure outlined below (you're already on step 2) and you'll get to deal with people who should have a more knowledge about the system. After 8 weeks you can then take it to Ombudsman. As compliant handling is a big thing in the industry right now, its been actively monitored by OFGEM, if you send something to the official complaints team it should be treated fairly quickly as it will be added to there complaints stats.
There is a phone number to call, but best practice is to send a letter headed 'COMPLIANT' recorded delivery and note down the times of any contact you have with the supplier. As your leaving the country I'd recommend calling the complaints team today asking them to put a stop on the account so no further letters/bills etc. are created until the compliant is sorted. As a stop gap you could pay some money up front to cover a few months worth of missed DD's. Not ideal for you but it should stop the letters whilst the compliant is investigated. You can also ask for good will gesture if its BG fault, won't amount to much though!
http://www.britishga...lectricity.html