If we knew that we'd probably all move there! It varies even from borough to borough in a big town. It took us six months to get from referral to diagnosis.
Then it depends what your area or borough offers in care options, how easily accessible and expensive they are, how rich your council is and how it is going to implement the new care act.
In general you might fare better in a bigger town, and an Admiral Nurse close by also helps.
Scotland has a slightly different system I believe but I can't remember the details.
I'm from Northern Ireland and I heard that we have one of highest diagnosis rates. Doesn't necessarily man the after care is great though but the cost of a care home among the lowest in UK. And any care visits provided in own home (up to max of 4 per day I think) are free unlike in England. Also housing etc quite cheap.
As has been said a large town preferably with an >>>Admiral Nurse<<< is probably the best choice for ongoing treatment. Large towns will have better scanning equipment available.
As regards diagnosis you will first have to obtain UK residency so that you can be registered at a GP, the next problem may well be getting the person to come out of denial and agree to be referred to a memory clinic.
I think the time scale then depends not only on the clinic but on how the person presents themselves, my local clinic has 6 month or 1yr recalls for most people but for my wife it was every 3 months from Autumn 2009 to Autumn 2011 after which I used to phone the consultant's secretary if I had problems.
It took 4 weeks from referral with a CT scan and MRI to diagnose MCI, in summer 2011 it took a further MRI and SPECT scan to reach a diagnosis of severe degree LBD.
There seems to be an increasing tendency for memory clinics to discharge people after a few appoints (maybe only one) and wait for them to be referred again if the GP detects new problems.
As has also been said the partial introduction of the Care Act 2014 in April is going to radically change things, also in my area (Greater Manchester) the health (NHS) and care (LA) aspects are being combined and will come under the complete control of the GMC (Greater Manchester Council) and 10 LAs. No idea what the outcome will be. Other areas may follow suit.
It took 6 months to get my mum a diagnosis from GP referral, which was undertaken in Worcester by my brother - the system there takes 6 months to do this as they send the CPN out a couple of times before referral goes to the consultant, and then he saw her a couple of times and she had a scan, from what I can gather from my brother they were looking for signs of stability/deterioration to ensure they diagnosed the correct form of dementia and there wasn't any question she didn't have dementia. (reading on TP not all types of dementia are suitable for medication). This was frustrating at the time but does make sense in total.
Other help comes from SS which as my mum is self funding I haven't really accessed, when we realised the issued I moved my mum into an assisted living flat (extra care) and they required an SS assessment to grade her care package, but then the case was closed.
Dayperson, there was a poster who used to contribute here who was based in France, and she was given a lot of support - GP and other - with her mother, who had dementia. Might it be worth, at least in the short term, trying to find a more sympathetic GP?
Perhaps you should make a determined effort to locate a doctor in France who is more sympathetic, you have said in a different thread that you father would be prepare to move anywhere in France or England if good treatment was available.
Thanks for all the links and suggestions. It's handy to know that she should be entitled to treatment in France and I'll have to look into all the possible options including whether we can get back to the UK quicker or persevere with the system out here. It's difficult juggling work with mum and for the last few years I've been struggling because she gets me upset with her behaviour. Then I have to pull myself back together and carry on and it disrupts me working which pushes back the date we can return.