Adding amber, biscuit, or home-toasting a bit of the malt seems to do a little something, however: nothing can get you that taste perfect, better to just get some MO.
No, you cannot simply add some Biscuit. It's not that simple.
Nothing can really replicate the flavor of Maris Otter except...Maris Otter. Many brewers, for reasons of economy, try this end-around. Those brewers might get close, but they'll all fail to a greater or lesser extent. Frankly, I don't know why they bother. It's a hobby; why cheap out?
If you want that flavor, go and get some Maris Otter.