Have a look at some of the Information Assurance products that L3-TRL are making in Tewkesbury - specifically Mini-CATAPAN LITE.
So... we are generally agreed that the lower ILs (Which is the new Cabinet Office speak for Impact Levels or security classifications/domains to us normal people) are not the real issue. I think that's what I said early before A2 Matelot violently agreed with me. The higher levels will always be problematic as it is almost always impossible to demonstrate that a vendor's equipment is completely secure (and functional for that matter) so everyone likes to make their own bespoke, customised and Heath-Robinson solution.
Some owners are so sensitive to this risk that they will not even recognise that there is crypto strong enough to allow their classified traffic to be carried on the same bearer as that of a lower classification.
However, it has been done before and is being done now. No system is completely secure, everything is susceptible to intercept, denial, disruption, misuse or insider threat. At some point you have to accept the risk otherwise you are paralysed and get nothing done.