> As to why the North did not secede I would probably say that when you have the majority of the votes you don't need to be the one to leave.
This was certainly true in 1860 and arguably for the better part of the decade before that. But abolitionists had been advocating for secession much earlier. They were a major factor in the Hartford Convention in 1815.
Also, abolitionism as a movement had the unfortunate but entirely predictable effect of making the South harden its own position. In 1831, the Virginia legislature was considering a bill to abolish slavery in the state, and it had a good chance of passing. Then word came that William Lloyd Garrison had started publishing an abolitionist pamphlet in Boston, and the bill failed.
> Slaveholders were a tiny portion of the UK economy by the time of the 1833 abolition act
The payments to slaveholders were 40% of the British Treasury's annual income and about 5% of British GDP at that time. Hardly negligible.
As to why the North did not secede I would probably say that when you have the majority of the votes you don't need to be the one to leave. The slave states were already losing ground in the legislature and after the 1860 census they would fall even further behind. The North could just wait them out and then impose its will, and the South knew this...