Modulation and demodulation of signal at modems and DSLAM(s) - including delay for convolutional interleaving / de-interleaving of symbols prior to modulation.
Travel across the BT Wholesale network to the relevant WBMC gateway where A&A connect to BTW via PPP sessions encapsulated in L2TP. Will be encapsulated and decapsulated multiple times potentially.
Decapsulation from L2TP at the A&A BRAS. I believe it uses LCP echos so nothing to encapsulate in PPP.
From the Highlands to A&A's network you're talking, give or take, 20-25ms across BT Wholesale. The rest will be interleaving, modem related excitement and any very slight delays either side.
psychopomp1 has BT Wholesale FTTP - his trip across the BTW network will be slightly shorter than yours as he's going straight to an NGA headend but you're talking a millisecond or two. The rest is xDSL versus FTTP.
HTH.