Thanks @Momasita, I have looked and it does look like you can make the books, save them, then purchase a code and redeem it later.
I have spent the last 45 minutes playing with various sites and software packages. In case it helps @Reastie and others, here is what I found:
Colorland, Photobook UK and Printerpix always seem to have photobook deals on Groupon. Photobook UK has dire reviews so I discounted them. CEWE offer £10 off your first book, but don't seem to discount much other than that. Truprint have 60% off photobooks and 70% of 60+ page photobooks until 1st Dec.
I downloaded the CEWE software, and it was a joy to use. Really easy to copy whole pages from one book to another if you're working on multiple projects and add photos etc. I tried making a photobook on the Colorland website but it was a little glitchy and each page, even without photos, took an age to load. Maybe an option if you don't have a big book to do, but I would die of frustration. The Printerpix webiste was much more user friendly and, whilst it would take me at least twice as long as it would with CEWE, was something to consider. I also looked at the Truprint website which I've used before, which was similar to Printerpix, maybe a little easier, but that could be because I am more used to the format.
CEWE charge £70.79 plus delivery for their basic 100-page hardback A4-ish sized book (£10 off first order)
Colourland have a running deal on their site of a 112-page A4 book for £10.99 plus £5.99 del and £2 to remove their logo. There are various Groupon offers around the same amount.
Printerpix's base prices are similar to CEWE, but they seem to have some sort of permanent discount running and are on Groupon too. £10 for a 100-page hardback book plus £4.99 delivery, cheaper if you buy 2 or 3. I think you get a further Groupon discount if you are a new customer too.
Truprint works out about £26.88 plus delivery for a 100-page book with their 70% promo (TREATBOOK70)
I still have Photobox to try out. They have 50% off photobooks at the moment, but it ends in 2 days, so too soon for me. My flash player didn't seem to want to work with their site, so I'll have to come back to that one later...
I am thinking that I'll probably start creating a book with Printerpix and see how I get on. It's too much of a saving not to try!