Edit: Interestingly today I get a figure of £558.51 from a dummy booking on BA.com. Which doesn't quite align with Ldnn1's £555.07 ITA figure. It's all so complicated!
£555.07 is quoted by ba.com for a departure now.
However, at some stage the UK PSC goes up from £41.47 to £44.91. And at some stage, APD goes up from £150 to £156.
So your dummy booking is obviously after the PSC increase but before the APD increase. If you dummy book for 30 March out, 20 June back, you'll get £558.51. If you dummy book for 12 June out, 20 June back, you'll get £564.51.