I'm all for acknowledging the errors made by everyone about everything, but apologising is too much to expect from people who clearly had no personal part in it. Who are they apologising for? Themselves? They had no part in it and doubtless find it completely abhorrent. The people who did have a part in it? Then that apologty is probably worthless as the people concerned had no contemporary problems with it, so doubtless didn't feel they were doing anything wrong.
If we want to bang on about the slave trade why not do something practical about stamping it out in the world today where it is still prevalent.People are trafficked and forced to work here. Children are prostituted against their will in this country, that's a form of slavery, so lets stamp it out. These links should get you started.
http://www.antislavery.org/
http://www.anti-slaverysociety.addr.com/
Surely the best way of honouring those we enslaved is by making sure we stamp out as much slavery in as much of the world as we possible can, not empty wordy apologies. Let's do something practical.