Chilli,
The applicable standard which the UK claims to conform to is ICAO Annex 11. If the MATS 1 does not reflect that internationally agreed requirement then it is up to the UK to either change the MATS 1 or publish a difference.
To use your argument, controllers could impose an ATC service in classs G airspace. i.e. it is above the minimum service!
The same argument whould also permit a pilot to demand a higher level of service than FIS in class G airspace.
Annex 11 gives a clear requirement for the country to select an appropriate class or classes of airspace to suit their needs and then provide the services appropriate to that class of airspace.
Thus while I would agree ATC could offer to provide a level of service that is higher than the standards required for a particular class of airspace (and as a controller working with only an ATZ and class G, you no doubt do on a daily basis), a pilot can decline the offer. At the same time, pilots can (and often do) request a higher level of service than the class of airspace requires to be provided and ATC are entitled to say it is not available or to provide it as they think appropriate.
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mm_flynn,
You don't have to go that far away.
France has class E for much of the lower airspace including airways, cta's and ctr's. The controllers there do not feel the need to control VFR flights on the airways who are talking to them just because they have IFR traffic.
Ireland on the other hand has taken a slightly different line and everything is class C. However, they at least have told everyone that they will (and do) separate VFR from IFR in controlled airspace.
Regards,
DFC