Sorry, Matt, that's just a meaningless over generalisation.
The Battle of Britain Memorial Flight (BBMF) has always been very wary of flying formation with any non-RAF aircrew and aircraft. That's been unit policy under a sequences of 'bosses' (BBMF Commanding Officers). Most mixed (RAF BBMF & other) usually involve the other aircraft flying trail or rear of a box. From the images I've seen, that's essentially been the case with the Canadian Warplane Heritage's G-CVRA Lancaster flying trail to PA474.
What I'd suggest is you're seeing is an adapted transit formation here - few, other than national display teams - fly tight formation from location to location. In my - now a decade old - personal memory of the BBMF they usually fly a lose vic in transit.
Yes, they might've tightened it up flying over the City of Lincoln*. There's probably good reasons why not however. I suspect - but it's only a guess - that they flew like this as they weren't prepared to fly the two Lancs any closer, and positioned the fighters to ensure the Canadians weren't flying 'behind & alone'. EDIT: Um, should've read the caption - C-GVRA is in lead, so above = wrong!
From the interwebs, here's some notable RAF formations. The Royal Navy and Army Air Corps can do some good stuff too. (Sadly we won't get to see the Royal Scottish Air Force's effort now.)
http://www.linton-gin.com/misc/EIIR.htmlPossibly my favourite multiple aircraft dissimilar formation.

Various versions available. This via Key Forum.
BBMF:
Standard formation at literally thousands of shows, for longer than
any other commemorative 'warbird' organisation.
http://www.pixstel.com/raf-bbmf-formation_urlb26662.phpSpecials:
http://www.airteamimages.com/avro-lanca ... 47258.html
John Dibbs
http://www.planepicture.com/news/latest/page:3Regards,
*By the way, City of Lincoln is also the proper name of Lancaster PA474, not 'Thumper' as widely used recently.