My recent DONE ex-ARN by AA was ticketed with 6,9% in taxes and fees (full 20 segments). It's higher than Gardyloo's number but the ITN has 2 LHR transits + 1 LHR open jaw, so taking that into account it's quite reasonable, I'd say.

AA STO (TAL) is highly recommended from my side also from service point of view, excellent knowledge and very responsive.