It's a really bad use of both miles and miles to upgrade from ET to CE! Especially to BCN which has the cheap I class routes from LGW for 200 quid or under return in CE both ways. You should only be able to upgrade YBH fares from ET and these are hugely expensive...this far out, I am almost certain it'll be cheaper for you to buy a Club (Cheapest) return than an ET return you can MFU one way.

If I understand your question correctly (which I'm not sure I do - what do you mean by 'the part that didn't rely on availability'?) I think you may be seeing some bug in the system...I would urge you to revisit the advice above though and just check in to it...