The US State Department has some pretty good information resources that detail what is needed to obtain and enforce an international legal action.
Specifically, the following came from the State Department site:
"PERSONAL JURISDICTION: It is fundamental that a court must have personal jurisdiction over a defendant before it can enter a valid judgment imposing a personal obligation on the defendant. Kulko v. Superior Court, 436 U.S. 84 (1978). In Pennoyer v. Neff, 95 U.S. 714 (1878) the Supreme Court set down the basic rule that a personal judgment against a nonresident defendant who was not served within the state, and who did not appear or otherwise assent to the jurisdiction of the court, is invalid. Over the years, however, the Supreme Court has substantially qualified Pennoyer to the extent that, under certain circumstances, a state court may properly acquire personal jurisdiction over a nonresident even though the defendant is not personally served within the forum state, provided the defendant has certain "minimum contacts" with the forum state "such that the maintenance of the suit does not offend traditional notions of fair play and substantial justice." International Shoe Co. v. Washington, 326 U.S. 310 (1945). See also, e.g.,
Hanson v. Denckla, 357 U.S. 235 (1958); Shaffer v. Heitner, 433 U.S. 186 (1977); and World-Wide Volks-Wagen Corp. v. Woodson, 444 U.S. 286 (1980). As noted above, foreign countries may find that the U.S. interpretation of this issue differs from local foreign law, rendering the U.S. judgment unenforceable abroad."
And since the US courts would not have jurisdiction over you in the UK, you should be immune to any US action.
Relevant US State Department sites:
Enforcement of Judgments
http://travel.state.gov/enforcement_of_judgments.html
Processes in UK
http://travel.state.gov/uk_legal.html