I think you are absolutely spot on there PB. I find that SRS just round the corner, relatively speaking - so I buy a fair bit of stuff there and my face is well known - give me considerably better prices than Euro or Pentland who are the other two biggies within easy striking distance. They all say they are giving me "trade" but SRS always come up considerably cheaper.
Euro can be cheaper if you buy online with a 'click & collect', but at other times, in-shop prices are cheaper. You never know, but next time you're in, have a glance around at anything on display, and check those items online when you get home. Sometimes wildly different.
Some time ago, when I needed some Ceratec grease, I ordered online, but at the shop, on display at around half the price. So I cancelled the click & collect items, and picked them up off the shelf.
CarParts4Less is Euro, and always cheaper, but seem to list a smaller range. Occasionally, just what you want is not available.
GSF are difficult. Their website prices are silly high, but most of teh time they run discounts, which can bring them down to reasonable levels. But, mostly they will not tell you what you'll get, preferring to describe as standard and premium. If you want a known brand name, need to shop elsewhere.
For me, Euro is only for 'need it now' items. CP4L usually despatch same or next day, so quite quick. If time is not important, Mister Auto is good, but Autodoc better. Mister Auto recently 'improved' their website, making it difficult to browse, and filtering the results seems impossible.