Mark it out using your mortise gauge.
Drill out the middles with a forstner.
Clean up the edges using a very sharp chisel.
Trying to bash through the entire thing with a mallet(or hammer) and chisel is both time consuming and so 16th century.
Deep mortises with a router can be tricky, plus you can end up taking ages having to stop, and wait till the bit has stopped before withdrawing. Vac out the chippings as no extraction is going to work adequately at any depth, then continue on. stop start stop start. More of a pain than anything.
The mortise machine is a great bit of machinery, and in industry its the go to machine for doors, beds, benches and the like. Bit of a drought on them currently. Axminsters sticking to the one big thousand quid affair, RP BM16 is a great home machine with its sliding actions, but is currently unavailable. Some of them just have a hold down which to me seems a let down and problematic in setting up - Jet being an offender here.
Draper and sip make their own versions(well actually dont actually make it, but you know what i mean) the sip one is better priced than the draper for effectively the same thing. If you've room and 400 odd quid to spare, id recommend that, and as long as you have enough projects in the pipeline to justify the outlay. Alternatively a bit 2nd hand one via ebay or ebay, but might have to travel a bit, and the big old ones are industrially heavy.
Took a look on gumtree for down your way and heres a cracker.
https://www.gumtree.com/p/other-power-tools/multico-hm-morticer-hm-1/1410973390
I'd reckon time wise bashing out the mortises if you were to equate to any sort of cash equivalent, £150 is a bargain.