The SSD in the early 2015 MBP uses a proprietary connector and also used AHCI (not NVMe). There are now SSD's from a couple of companies - the OWC "Aura Pro X" and the Transcend JetDrive 820. Both should fit into the early 2015 MBP. I still have not seen an in-depth technical review on either of these products so I don't know how well they work. The OWC and likely the Transcend require High Sierra. You may also be able to use an adapter and a standard PCIe NVMe drive like the Samsung Pro 960 (would not recommend the 960 Evo). This would also require High Sierra. There is along thread on this at:
https://forums.macrumors.com/threads/upgrading-2013-2014-macbook-pro-ssd-to-m-2-nvme.2034976/
The title says 2013-2014 but deals with 2015 as well. Post #494 on page 20 has a poster who has used the 960 Pro in a 2015 MBP.
There are other lower-performance options if you don't want to use High Sierra. As mentioned in post #3, one can also buy a used SSD, however, at this point, I would be weary of these since one does not know (unless the seller discloses it) how many erase/write cycles the SSD has.
I see on Amazon UK that the Samsung Pro 960 1TB is £530. (As mentioned earlier, this requires an adapter.) The OWC Aura Pro X is $650 here in the US. I would consider that kind of pricing as the base price for a new, good-quality likely-to-work solution. You could spend less and that would likely involve a compromise - for example, the Samsung 960 Evo is cheaper but uses more power and generates more heat, the Toshiba/OCZ RD 400 SSD is a very good SSD (also requires an adapter) - a 1TB version is on Amazon UK for £458 but you'd have to do research to see if others have been able to make it work with the adapter in the MBP.
In the US, Apple charges $600 to upgrade from a 256GB to 1TB SSD in the 13" MBP. So you can figure out what's best for you. If you buy a SSD for the 2015 MBP, whatever it is, it cannot be used in the current or 2016 MBP - that may be a consideration for you. (It almost certainly will also not work in future MBP's.)
