Your intial post indicates you are purchasing small lots...packs of 12 which are generally always going to be more costly than case lots.
Here in the States we are lucky to have a number of on line sources for a wide variety of diapers availbable in case lots, which cost a lot less than smaller packs of 12 or so. Shipping does get added on but often it is still cost effective to get a case and pay shipping compared with buying small lots in the store.
But, before you spend money on a case lot, you need to know that is the diaper for you....perhaps two types of diapers considering day and over night needs. There is nothing worse than investing in a case of diapers that don't do what you hoped they would.
You may be restricted with availablilty but research on line to see what sources you can find with diapers that will work well for you and shipping will not be prohibitive. It may be worth it for you to read up on diaper testing at
www.xpmedical.com even tho you probably won't be able to buy from them. They do a great evaluation of a lot of good diapers and break it down to cost per oz of liquid absorbed which is a real way to evaluate a diaper, in my opinion. Just as you said, it can be cost effective to wear a more expensive diaper that you change less often.
Ofcourse, your skin sensitivity can effect how long you can tolerate a wet diaper as well as how you manage to hold the diaper up thru the day with activity and the diaper getting wetter and heavier...this is where I refer to the suspender briefs as a consideration (See the Primer addendums).
If you can't go 8-10 hours in a wet diaper, for whatever reasons, then the higher cost diapers won't be cost effective and a mid range performing diaper at less cost but changed more often could be a cost effective solution.
I do think XPMedical is a good source and you may find diaper brands there that perform well and are available to you in the UK. Tena and Attends come to mind as high perfoming diapers in the Euro version even tho they are not great in the Amlerican made version.
Thank you for bringing up a good point with cost effectiveness for those who must rely on disposable diapers. If you can manage the hassle of cloth, you can use cloth diapering over night to effect substantial savings with using disposables only by day.