The CAA classifies ex-military aircraft into three categories:

a) Simple: single piston engine types.
b) Intermediate: multiple piston engine or turbine (single or multiple) engine types with simple mechanical flying controls or with powered controls having an independent back-up system that can enable continued safe flight following failure of the powered system.
c) Complex: all other types, including those having features that require specialised knowledge and/or equipment to maintain, aircraft without independent back-up systems for powered flying controls or having automatic stabilisation systems or electronic engine controls.

The first two categories are relatively easy to get onto a Permit to Fly, but the Complex category is the one which requires continued airworthiness design support from the manufacturer or an equivalent organisation. At this moment the Vulcan is the only Complex type which has managed to get over this hurdle. In this case they transferred this support from BAe to (IIRC) Marshall’s.

For the interested, have a read of chapter A8-20 (para 1.2.1.) and Supplement 1 and 2 to A8-20 in this document: http://www.caa.co.uk/docs/33/CAP553.PDF