If Harry were captured would there be a special forces attempt to rescue him? Of course. But that wouldn't necessarily be bringing new pressure upon the Taliban. US and British Special Forces have already been operating heavily in AfPak ever since 2001. And it wouldn't necessarily be successful. It took us a decade to find Bin Laden. If the Taliban were able to smuggle Harry across the Durrand line (not that difficult), that would make things very problematic. (on a side note, the capture of Harry would be a very interesting snag indeed. I have no clue what the Taliban would attempt with such leverage. I don't think they do either).
If Harry were killed, would Britain and/or the US suddenly reverse course and prepare to "surge" again. Of course not. First of all, why would it? The thousands of soldiers killed and wounded in Afghanistan and Iraq haven't fueled public desire to step up the war. On the contrary, the opposite is true - it's slowly but surely eroded public support of the wars. I see no reason to believe that a high profile death would have the opposite effect. It would only further reinforce the frustrated public sentiment that "we need to get out of afghanistan ASAP". Of course this is mostly based on my perception regarding the American public, but I frequent England often and for extended periods, and I'd say it's roughly true there as well.
Secondly, to what end? What would the mission be? To kill 1000 Taliban as vengeance? The Taliban have already been removed from power (official power, that is). And a simple ramping up of forces has already proven to be insufficient to completely eliminate the Taliban, given their, ahem, foreign support. And no one's going to war with Pakistan, however loved Harry is.
The Taliban aren't stupid. Their strategy has always been to undermine morale and public support for the war. This is why they seek high profile killings and bombings - they make headlines and reach the living room TVs of Americans back home. The Taliban have always been banking on the US public not having the stomach to see the war to the bitter end. Best I can tell, they were right.
I would also like to point out the I believe the threats are mostly hollow. The Taliban are hardly in a position to go around picking off specific targets of The Coalition. If they nabbed Harry it would be a result of random dumb luck, not effort.