Container grown apple trees - variety and rootstock

I would choose rootstock M9 (dwarf) if possible, or next one up which is M26 (semi-dwarf), or one down which is M27 (very dwarf). The smaller the rootstock the more care you'll need to be prepared to give. A semi-neglected M26 will be "dwarfed" by the neglect and behave more like an M27.
Regular watering in summer will be essential for the plant health, happiness and fruit production (a pint of water once per week).
Most people seriously underestimate how much water a potted plant needs in summer and how a pot containing pure compost is difficult to rehydrate once it is dry; water runs straight through and the plant doesn't get to absorb it).
But don't water too often or too heavily as it can encourage root rots. The dwarf rootstocks mentioned (M27, M9, M26) have some resistance to root rots.

As for choice of variety: there are many to choose from but I find most of the Cox offspring to either be disease-prone or the fruit flavour falls well short of my expectations after reading all the sales pitch.

There are many Cox offspring. At the bottom of this link to Keepers Nursery is a long list of links to Cox offspring:
Cox's Orange Pippin apple trees for sale | Order online

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Thanks for the advice FB. One question about it though, would 1 pint a week really be enough for an apple tree in a container over the summer?

If you water too much (and the soil is saturated) you risk causing crown rot - even rootstocks with some resistance can become infected if exposed too much.
Also if you water too much you end up with lots of leafy growth and fruit quality might suffer as the shots steal the nutrients from the fruits. Bitter pit is common if there is too much nitrogen fertiliser, too much water and too much growth.

The best fruit is from a plant which is slightly hungry and slightly thirsty - but not starved!

I'd probably be inclined to use M26 rootstock and use three-quarters garden soil with one-quarter compost in the pot. I would then give half an inch mulch of compost once or twice every year as feed, and not give it anything else unless I felt there was a problem. In that way you may not need to change the soil often because the compost mulch will give a slow-release of nutrients.
In the very early years if you need more growth give more nitrogen.

To start with, a pint of water per week in summer will be enough because its root spread won't be very far. But if you eventually have it in a half-barrel you might want to evenly water with two to four pints. But beware of causing over-vigorous growth if you want good quality fruit. You either get good growth or you get good fruit: you can't usually have both (although the very vigorous M25 rootstock seems to be an exception to the rule - but is not ideal for containers!).

Thanks for the reply FB, very useful comments.

Growing fruit trees in containers and an easy life are mutually exclusive!