Addition to my story above: the next week, on Friday morning, I went to Harrods to check on the Lindy26 stock. No available, even for Lindy30. I asked the SA to let me know which shop shd I go instead visiting all the other 4 stores. The SA was not allowed to tell me any inventory of other shop so I was grateful when she's hinting me to check NBS first instead Sloane tho Sloane is much nearer. She did hinting 26 might be in exotics only so I shd keep my option open to 30.
I reach NBS around 1130 noon. The SA said seems there was L26 in system but after checking, she said it was unavailable. So I asked for L30. At the same time, there was another girl who looking for L30. So the 2 SA discussed, went inside and came out with 2 boxes. They called us both and opened the 2 bags at the same table lol. I was offered black clemence, she was offered blue sapphire swift. I like clemence but i have too many blacks, plus i really love blue sapphire. I told the SA, i will sure take 1 bag, but I'll wait the other girl decide because I honestly prefer the blue sapphire. And the funny thing happened, the girl says she prefer black

and that's it, we swop the bags

everybody's happy. IIRC the clemence was GBP 4570 and swift GBP 4620. Tax refund in NBS: 13%.
It was my first hermes bag and I would say I'm very satisfied with the service from all London's SA in all the shops ( except Royal Exchange that I didn't go in). Very polite and helpful even I am first timer and no, I didn't dress up at all. But as I learnt from other thread, keep positive and polite to the SA.
It wasn't K or B experience but I've gained alot information from here so it's time for me to contribute as I know looking for L could be hard too nowadays. Well at least I know my husband, sister and cousin have been trying for me since April 2015 whenever they travel to Europe and they always got No for L.