Another 20 years? The man was 80 when he died! (Pretty spry for an 80 year old but still you want him to live to 100!!??). However, on the foreign policy front Anglo-American relations would be interesting. He'd probably be very reluctant to consider the Alabama claims, and he'd certainly not draw down British troops in Canada, while adamantly decrying the Fenian issue in North America (one more battle Johnston would be destined to lose). He'd crack down pretty hard on Fenian violence in Ireland (I think he wanted to suspend trial by jury in the case of a rising). On the foreign relations issue, IIRC he was pushing for closer relations with France near the time of his death to try and counter Bismarck's ambitions in Europe, while also probably condemning the Russo-Turkish war (if the Liberal Party is still in power at this time).
So you probably have more of a 'hardline' British foreign policy until 1874, with closer relations with France, and frostier ones with America.