"The Register understands the transition slashed costs by hundreds of thousands of pounds, halving monthly user fees from roughly £25 per month to £12 per month."
This says a lot more about the raging inefficiencies within Parliament than it does about Office 365 (especially since Google and MS's big selling point about these services is "It's FREE!")
The security ramifications of all parliament's mail being on servers in Ireland don't bear thinking about - and similar experience with outsourcing this shit shows that what you may save in upfront costs you lose in added support costs.
The only valid reason to outsource this kind of thing is in order to declare the existing ISD management structure redundant and start over with competent bods, then pull it back in as rapidly as possible.
FWIW, now that Goo's dumped gmail for ISPs, I expect to see gmail for businesses go soon afterwards - and the only reason that MS even offers this service is because Google does. Just remember that if the service is free then you're the product.