I've had a few recent interactions with the CAA and they've been OK. Maybe a little clunky, but nothing to complain about.
A change of address came back in 3 days for my license but it took about 10 days for the aircraft. Their processes could be streamlined. For example, after you put in a change of address form for an aircraft, then they write and ask you for the certificate of registration to be sent back to them. Only after this will they issue a new one. Not sure why they needed the old certificate for the aircraft when they didn't need the old license from the pilot. I guess aircraft and personnel licensing are different departments. They're also inconsistent between allowing you to make applications on the web in some cases, but requiring other things to be sent by post. A bit of clean up of admin processes wouldn't do any harm here.
For the IMC it was nearly three weeks, though I did follow up on a tip from my CFI and rang them at Gatwick to confirm how much the courier return of license and logbook would cost before I sent it all off to them. I added the courier fee (about six quid) to my cheque and it all came back nicely. I even got some friendly advice where someone pointed out instrument hours I'd not put in the instrument column, so they'd clearly checked it all through properly.
All in all pretty satisfactory. I think there's some opportunity for removing some admin redundant steps to save them and us some time, but it was a damn sight easier than dealing with banks or insurance companies.