You might find something like the Nobex Mitre Saw will do the job NOBEX.
Cost about £100 though. This will enable you to cut the skirting up to 180mm deep.
Last time I did the job I used a compound sliding mitre saw - as you are limited in depth of cut you will have to lay the board flat and cut across the depth of board.
From my limited experience I believe it is unlikely that you will find any true 90 degree angles either external or internal. For internal angles I believe it is better to scribe the joint.
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