I think the best (though perhaps not the easieist) way to approach this question would be to consider the manifestos of the two parties over the last 2 or 3 general elections and try to find the common policies. So if, say, in 1997 the Conservative manifesto had a policy on economic spending, compare that with their 2001 and 2005 policies on the economy and see how much change there has been. If none, that would be a "distinctive political prinicple".
Of course, this may be tricky if you are unable to get a copy of the manifestos, and anyway what is in the manifesto may not (in the case of Labour who won the last three elections) be the same as what is actually done (not a political comment per se, there are several reasons to do things differently from how you said you would do them).
If that proves too hard, then instead you might want to focus on two or three key policy areas, say the economy, crime and education, and look at what each party has said (Tory) and done (Labour) on these over the last decade.
I can think of one website that may help with this, but the main source would be political news columns that may be accessed from newspaper websites such as The Telegraph (useful if you want to know what the Tories are saying) and the Independent (whose archives seem to be free-to-access). Try avoiding the "comment articles" since these will be opinionated.
three60 (of Cambridge
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