Ryan and Jade Dash, both 31, did it in 15.5 hours — with the added rule of using a different mode of scheduled transport for each leg.
Starting in Cardiff, they got a bus to London, then travelled to Edinburgh by train before flying to Belfast.
But disaster nearly struck when their flight was delayed, putting their goal in jeopardy.
Jade said: “It was the last flight out and if we missed it, we had no chance of completing the challenge in 24 hours.”
The couple, from Manchester, documented their trip with videos, which they put on social media.
They added: “People think adventures lie in destinations far away, but they can happen on our doorsteps too.
“It was a fun way to see the UK and even though we got limited time in each city, we did manage to enjoy seeing the sights before we left.
“In total it took us 15.5 hours to complete, the timings of the public transport mainly dictated the way we did this – as the rule was that we also had to use scheduled public transport.
“The challenge was hard as we were constantly on the go and with such tight timings any slight delay could stop us from making it.
“We really enjoyed Edinburgh – it’s picturesque and we were lucky enough to have a blue skied day which we’d been told was quite rare.”