I have been remiss in not mentioning Andrew Hill's blog, 31p purple. It's about modern GB definitives, and it's another Machin presence on the web (always a good thing).
Hill has been collecting the Post & Go issues, but recently he has thrown in the towel. I'll just quote a paragraph here, but I suggest you read the whole thing (his illustration is well worth seeing).
My main frustration is that I cannot see how I can ever get anything resembling a complete collection as new denominations get printed on old stock so you can never close the file on a particular denomination or background as, at any time, a variety may emerge either by mistake or from a distant office with slow-moving stock.
Of interest to me is a statement he makes, "If I had seen more of these in actual use that might have convinced me to stick with them..." That mirrors comments I've read elsewhere that for all the machines available, these labels are not used very much by the general public.
Considering the number of pictorial issues, plus the number of overprint varieties coming from short-time locations like stamp shows and commemorations like the Penny Black's 175th anniversary, we have to wonder whether the primary purpose of Royal Mail's Post & Go has changed from public convenience to revenue generation. Certainly that's been the rationale for Jersey, Guernsey and Gibraltar, who have issued Post & Go stamps at U.K. stamp shows when there were none available for the local public to use.
Other posts worth reading are Hill's comments about the Machin pane in the Battle of Waterloo prestige booklet, and his explanation of what a "Concrete & Clay" booklet is.
Besides the blog, Hill also created and maintains "The AHI Machin* Catalogue." (The asterisk indicates that some non-Machin items are included as well.) In his introduction, Hill states, "I have tried to produce a listing which includes all the issues which have been officially recognised by the British Post Office since 1967 and which features all the changes which produce items which can be reasonably readily identified as different."
If you are looking for an intermediate-level list to guide your collection, Hill's catalogue is worth investigating.
Note: The title Post & Gone is hardly original. Brian Sinnott used it for a different topic on his blog, Postage Labels of the UK.