I have found that there is often some 'Service Drift' with the type of service you receive, but don't count on it.
When flying in the Midlands, maybe tracking towards a VOR, I have been given Traffic information even though I only asked for a Basic service.
The only time I have ever asked for a Traffic Service, was when undergoing instruction in IMC conditions. If I was given some Traffic Information, I maybe would have then asked for some De-confliction advice.
I don't know what range they use when deciding to offer Traffic advice, but it seems to be about 3-5 miles, which with two planes on a closing heading, gives you about 60 seconds warning of the other aircraft.
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