Is the House of Lords being used as a procedural tool by the Executive, so that “originating in the House of Lords” is not what it at first seems (ie a Lord proposing a Bill that becomes an Act)?
Sort of. Acts of Parliament (apart from money bills) can be introduced in either house, and for the most part, there is no particular significance to where a bill starts its passage through Parliament.
The Government controls the schedule in both houses (even though no party has a majority in the Lords), and so to balance the houses' workload over the course of a parliamentary session, it introduces bills in both houses, some at the start of a session, and others later on.
Was I wrong to think only a small proportion of UK law originates from the Lords?
Yes - and note that most Acts "originate" with the Government, rather than any particular individual (though many private members' bills are introduced, with most not becoming Acts). Which house they use to start a bill's passage is (mostly) a matter of scheduling.
From a comment:
Is the procedure that the Executive finds a Lord willing to sponsor a Bill to kick off the process?
It's not necessary to find a "willing" peer (Lord); each government department already has ministers who are also peers; see the names marked in red here. A minister from the relevant department will introduce the bill, just as happens in the Commons.
[Is there] a mechanism for Lords Private Members’ Bills too?