A population of 500k isn't "poulty". It's very sizeable.
Newcastle would be the usual suspect. Frequently listed as the top party city in the UK and one of the top in both Europe and the world, it is famed for ts nightlife. Its nightlife was voted the UK's number one tourist attraction by Lonely Planet(?) A bit baffling in my opinion as there are many better things about Newcastle and the UK in my opinion but there we go.
Its population is 320,000 or so but this is misleading as the population of Tyneside is the best part of a million. Added to this Newcastle's popularity as a nightlife destination brings in people from across the region and country every weekend. On a Friday night trains are frequently packed with people travelling from Manchester, Liverpool, Edinburgh, Glasgow, Leeds and "down south".
Its clubbing scene arguably matches Manchester's at the very least. Although it has a vibrant music scene, with plenty independent bands, it probably doesn't offer the number of small medium size venues that Manchester offers (or did offer five or so years ago) although it still has a good number in the city centre and Ouseburn area (a former industrial area to the east of the city centre which has experience regeneration and his a "creative" area with a couple of good music venues and also popular with artists.
It also has late night shopping in the city centre now, which is something some other cities don't have, and this has helped keep people in the city centre between those hours of finishing work and going out later, or encouraged those who otherwise would just stay home to come out, and this has provided a boost to the city centre economy.
But, really, I think any other large city (Glasgow, Leeds, Liverpool, Sheffield, Bristol, Edinburgh) will provide sufficient entertainment.