I'm a bit hamstrung with events because (before I got into audaxing) I'm signed up for a big Sportive in late April. If I do, go for it, I've only really got a choice of two 600s. Frankly, seeing as my old mum lives in Bedford (and is therefore a handy place to stay beforehand), the Buzzard was looking more appealing - and is on with more hours of Sun, but failing that, yes the Flatlands is my only other real option unless I really want to pee of the missus and do lots of long rides close together. Essex isn't too far from Bedford either but Leighton Buzzard is just down the road.
I would ditch the Sportive now if I could to be honest but I've arranged to do with others so I can't really back out, otherwise I'd be looking at the BCM and Brevet Cymru.
There are the permanents of course but I'd prefer to do the early long rides with company and potential help/advice if I can.