Pretty much any event in the UK (apart from PA League) can be played as a 'stand alone' as most tourneys give trophies and prizes at each leg of their series.
The Welsh open, (24/25 September, provisional) tho considered a 'stand alone' is part of the UK/Midland Masters.
Heavens Gate (23/24 July, provisional) also considered a stand alone is part of the SWPL
Then there's Campaign Cup, of course.. (part of the NPPL(?) September, Manchester, tbc)
As far as true UK stand alones i can think of the Zap Euro Indoor in Birmingham on 5/6 Feb, the Euro 5-man event held in a nice stadium in Ashford, Kent - usually October. Then there's the Severe/Atmosphere/JCS cup (6 November, tbc). The new Canary Wharf Indoor site was talking of hosting a few tourneys too..
Hope that gives u something to chew on