I'm wondering if bodies such as the U.S. Senate, or particularly the U.K. House of Lords meet year-round, or do the Senators/Lords simply live in the capital the entire year? What about historically, pre-industrial revolution? How did the government function when/if the Senators/Lords went back to their homes?
No, legislative bodies typically don't meet year-round, at least not in the US. There are recess periods built in to the calendar, during which members are able to both take vacations and go to their home districts. The longest break Congress has is a month-long recess in the month of August, but there are other scattered recesses throughout the year. When they are in session, members can still take breaks of their own, as long as there's nothing they particularly have to be there for. In the past, this was harder, because you couldn't get home for a weekend if you were from California; then, you still had recesses to get back to your district and talk with constituents.
Congress is not the government; they actually have very little to do with the day-to-day running of the government (they make laws, they don't carry them out). As a result, a recess doesn't actually cause much of a problem with the government's operation. This is also one reason recess appointments exist -- if a vacancy in the executive branch needs to be filled quickly and the Senate's not in session, the President can just appoint someone, precisely because that's faster than waiting for the Senate to return. Other than appointments, I actually can't think of any urgent and unexpected duties for Congress; declaring war may count, but the President can send troops before the declaration of war.
So in practice, legislative recess isn't an issue -- the legislature doesn't run the day-to-day operations of government, and so they can take a break without affecting those.
No, they don't sit year round. Helpfully both the House of Commons and House of Lords each have a web page where they publish their recess dates.
The Commons page is at http://www.parliament.uk/about/faqs/house-of-commons-faqs/business-faq-page/recess-dates/ and the Lords page is at http://www.parliament.uk/about/faqs/house-of-lords-faqs/lords-recess-dates/
There is some overlap between the recesses but they are not identical. For example, the Commons normally has a short sitting period between the summer and the start of the party conference season (in September and October) while the Lords does not; the Lords does not start its summer recess as early. Overall, however, they sit for a similar number of days.
Houses normally sit Monday to Thursday but only sit on Fridays on some occasions. The convention is that Fridays are devoted to backbench legislative business.
Members of the House of Commons who are not London-based normally return to their constituencies outside of the Monday to Thursday sitting period and for recess to meet constituents, go to events, campaign, or more conventional activities like see their family and friends. Members of the House of Lords do not have constituencies but may still return back home over the weekend or recess.
Depending on travel time, many MPs and peers rent or own a place to stay in London as well as in their constituency. Both houses can sit quite late in the evening.
The government remains the government whether or not Parliament is sitting. MPs and peers who are ministers might stay in London longer than backbench counterparts to carry out government business.