The reaon I say this. Theres numerous complaints on here. Its a poor experience, Giffgaff don't seem to care about the data experience so maybe its time they give up offering data and offered a core calls and texts package.
Anyway I did some testing. The test location was Manchester Piccadilly (Fairfield street lift entrance near the metrolink bus replacement), All same test server (Broadband 4 the rurural
Virgin Trains Wifi
Stagecoach bus free Wifi
Now I must be honest. I didn't get a screenshot of the Virgin Trains wifi but the download was 5mb down and 3mb up. (Platform 9 was the closest train to Fairfield street)
So Stagecoach bus here are there results.