Personally I'd go for Shropshire of those options. Its fairly central and has good access into mid-wales (mention wales and it'll kick off as usual
). I wasn't aware of it as prime biking territory but its nice up there.
If the alternative is lincolnshire I can't think of a reason to go at all
You know welsh borders (i.e. Symonds Yat, Ross, Wye Valley way) was always a good sugestion I reckon as there's loads of different areas to hit in a day but that never took off as an idea.
If its a long weekend I think the idea of a number of sub-group rides is a good idea rather than the normal slow trail around. Ie. we saty in one spot rally-stylie and then people opt for rides that they like the sound of.