Great buys:
A great, no frills baby monitor is the Fisher Price 'Light n' Sounds' analogue monitor. About £35 from Amazon.co.uk for the version with 2 parent units (have also seen a since parent unit one), has a night light on the base unit. It never drops out / loses the signal and base unit has a very sensitive microphone.
Sleeping Sacs / Grobags. If you're lucky and have a TK Maxx close-by, they may have some for c. £13 (half RRP) {conversely - flat sheets and cellular blankets for the cot have gone unused; DD went from swaddling wraps straight to gro-bags}
Widgey pillow for b/feeding
Tommee Tippee bottles & steam steriliser - DD was combination fed from 4wks and we didn't have the luxury of a dishwasher (have since moved and bottles, dummys, etc. still go in steriliser)
Baby Bjorn carrier and bouncy chair (££, but was present from MIL
). Carrier was a godsend for the 4wks early on when being upright and held close was the only way to sooth DD to sleep. Bouncy chair has been great from the start, but is now coming into it's own that DD is 6mths and can bounce herself.
We live in west London, and while I love my iCandy Peach travel system for everyday walking, I also got an OBaby Sport stroller for travelling and those days when tube travel is necessary. DH thought £80 was mad for a 2nd stroller, but this was the same cost as the iCandy travel bag, and after a week's holiday in NYC he agreed with me 
Useless purchases:
Cot-top changer. Never even took it out of the plastic wrapping
And DD hasn't had as much use of the Bumbo as I thought she would. Always seems happy for the first 5mins, then gets whiny and frustrated - and transferred to the BB bouncer