Presumably they are not in touch with ATC otherwise the benefit of surprise is lost.
The purpose of these flights are for training and to probe the defences of States along the way (for the Russians own intelligence purposes). Hardly a surprise that they don't come announced or file a civilian flight plan
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At what altitude do these interceptions take place?
The Bear is a large turbo prop aircraft. It's service ceiling is around 10,000 Metres.
Do these flights obstruct flights from North and Eastern Europe flying northwestwards to the Americas?
It depends on exactly where they fly and at what altitude. But potentially they can get in the way.
How does the European parliament view surprise military thrusts at its borders?
The flights all take place in international airspace where military aircraft (including NATO and European air arms) can and do operate as they wish. It's called 'Due Regard' and basically they don't worry about any other civil flights other than to not hit them in the interests of self preservation. For the civil controllers it is unsatisfactory and poses a perceived danger to civil operations, but it's legal.
If UK airspace is being infringed, why isn't the European Parliament onto it?
UK sovereign airspace is not being breached. Only airspace for which the UK has civil ATC responsibility or military air defence responsibility. It's no worse than the constant infringements in the Middle East civil air lanes by US or UK aircraft operating under 'Due Regard'. In some ways it's actually a lot less dangerous since the Bears take a long time to get to the airspace involved and are tracked by the military who keep the civil controllers advised.
Why is it not viewed as an unfriendly act to have such military probing flights these days?
The West do the same in other areas. We can't claim any moral high ground.
I just find it slightly unsettling having apparently unannounced and uncontrolled military flights flying in areas where I have flown airliners- or do they have their own corridors for this sort of thing.
As above, they go where they want, where they want. It's not nice to watch it from the ground either, but there's nothing that can be done other than to vector civil flights away from the problem areas.