Why not get the train up from Newcastle ?
If you book now you and your lady friend will get there for £37 (thats both of you not each) , saves driving and you can have a drink . We usually get the 9.36am (arrives Edin 11.06) then get the 7pm back from Edin (arrives N/cle 8.43pm) . More than enough time to get fed , watered , have a scout of the shops and do the heritage centre .
Regarding the purchase of a bottle , don't know what your tastes are but if you like peated what about the :-
Ardbeg Very Young
Laphroaig Quarter Cask
Lagavulin 12yo (slightly above budget).
At Cadenheads see if there is any Campbeltown Loch 21yo left , it's a blend but very good .
2 other shops worth having a look at are Jenners (the whiskies on the 2nd floor ), they have a good selection of G&M as well as others and Harvey Nicks , they have a slightly small expensive selection but they do (or did)keep a range of Adelphi bottlings .They also have the Islay Fine food meats .......
Have a look in The Whisky Shop in the Metrocentre to give you a better idea on what might be available.
If you ever go up into Northumberland , more precisely Holy Island , the mead centre has a cracking selection of malts
(http://www.lindisfarne-mead.co.uk/Shop/ ... election=6
) , again mostly G&M but i picked up a few older bottlings i'd never seen , a cracking 1991 Cask Caol Ila from sherry butts that if you blind tasted it you would say it was from the south shore of Islay.