You can't count on Jessops in my view and in all probability you'll get to talk to someone who isn't real sure what 120 is. Stock levels will be low, so the more you want and the more specific your need the less likely you are to get it.
By far your best chance of getting what you want lies in mail-order where you can place the order by phone (you can do it online too but that way you don't get to know about stockouts). I'd expect all these places to be better value than virtually all retailers, but their prices may not be the same.
Speedgraphic 44.(0)8453 305530 or www.speedgraphic.co.uk will post the same day if you order in time.
Discount Films Direct 44.(0)1623. 491746 or www.discountfilmsdirect.com also stock a wide range and are efficient.
Mailshots 44 (0)870 766 4151 or www.mailshots.co.uk are also good.
Don't use 7day shop- not that they're bad but they are not fast. Some people think they were so called because they're open every day, but rumour has it that its because it takes them that long to fulfil.