Hi Gal. As you said, LY uses a several a/c types for same routes.
For example, LY sends EVERY a/c type to CDG: 73G, 738/73H, 744, 752, 762, 763 and 772.
Regarding LHR: mainly 762, 763, 772, 744
STN: mainly 738, 757, 767 but sometimes 777s
Central and Eastern Euope receives mainly a mix of 737NG and 757s (but GVA can sometimes receive 767s, 777s or even 744s as charters...)
AMS: 737NG/757/762/763/777
NY: 744/772
LAX: 772 only
YYZ: 762/763
Beijing: 762/763
JNB: 762/763
DME: 737NG/757/762/763/777
HKG: 772
BKK: 767/772/744
MAD/BCN:737NG/757/762/763
MUMBAI: now 772s as far as I remember
...
Hope this helps
Correct me if I'm wrong.