I guess giffgaff and SMARTY are similar in being a no-frills alternative to their parent mobile networks (O2 and Three UK respectively).
I've used giffgaff previously AboveAndBeyond, but switched because giffgaff don't offer their customers a Wi-Fi Calling service. I rely on WiFi Calling at home living in a mobile not-spot. Away from home, I haven't noticed much difference between the O2 and Three UK mobile networks.
You'll need a 4G capable handset with SMARTY from 2025 onwards, because Three UK are currently switching off their 3G mobile network, with this work scheduled for completion by the end of 2024.