I’m looking at breathing new life into a 2015 27’’ iMac. Sadly, the machine has Thunderbolt 2 ports which means connecting it to a fast external USB C SSD seems to involve a great profusion of dongles and cables (which I don’t have and which will cost around £80).
I’ve had a quote for £260 to replace the fusion drive with an internal 1 TB SSD.
Worth it?
YES! I had the same scenario on a 2017 iMac and it instantly made everything feel twice as fast. I was constantly getting beachballs on the fusion drive and now almost never get that.
One small thing you’ll need to plan for is where you’ll end up installing your OS afterwards and if you want to re-fuse the new SSD with the internal small “blade” SSD that was originally part of the fusion drive. I ended up keeping them separate since the blade 128GB SSD runs faster so I put my OS on that and then my home folder on the new SSD. This might not work if you had a smaller fusion drive and your blade SSD is smaller as well, in which case putting everything on the new SSD or re-fusing might be better.
Apple’s Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter is only £49
£260 seems like a lot of money to spend on a 2015 iMac with a current resale value around £550
True, but still better cheaper than a new iMac if you’re just trying to get some extra life out of it.
£49 will get you there too. If you want to spend the money, spend the money, but don’t expect to get very much if you ever try to resell it - it’s already 5yo and will be close to valueless in a couple of years with the migration to Apple Silicon. Better to have an external SSD you can move to a new machine IMO.
I’d look at whether an ssd on this machine makes much sense. It’s pretty late in the game anyway, and will probably only survive 1-2 more OS updates.
A LaCy rugged thunderbolt 2 (or 3) is cheaper and would do the trick, I had my iMac running from there for 2 more years before it died on me.
I’m not sure an around 300 pound upgrade is worth it, seeing as the machine is still functional with the Fusion drive in it…
The only iMac left on OWC’s website is 2014, fusion 1T drive, 8MB ram, at $1739 (£1323).
My current 27" 2015 iMac has a 2TB Fusion Drive hard drive and I currently have about 2/3 of the HD in use, while the newest 27" iMac released earlier this week (https://www.apple.com/us-hed/shop/buy-mac/imac/27-inch) has only a 512GB SSD. Why does the new one only have a quarter of the size HD that my 2015 does? Does the new SSD HD have some witchcraft that compresses files that the older Fusion Drive does not? Or would I need to buy the new 27" iMac with a 2TB SSD?
In a few words, entry-level pricing.
Yes, you would need a 2T SSD to store what you have now.
Another thing that people do is buy the base-model 512GB drive, then use a fast external (Thunderbolt 3, or USB-C) to store their data.
E.g. this Sabrent 2T, TB3 drive.
https://www.amazon.com/dp/B07XQKRVLZ/ref=dp_cerb_3
Got it. Makes sense. 
I don’t recall if the 2015 model I have started with the 2TB or if I upgraded it at purchase.
The external disc you shared looked good but the price is relatively the same as just upgrading the HD of a new iMac via Apple at purchase. If I’m spending ~$2,300 on a new machine, paying a little more to get the right sized HD at purchase vs an external drive is the way to go in my book. May as well have Apple take more of my money. 
Yep! And it’s all covered by AppleCare.