oven under boiler

there is nothing wrong with this. modern boilers can be installed almost anywhere. I'm assuming here that the single oven is fitted under the worktop and the boiler is sitting above the worktop, in that case its fine. just watch out where the gas pipe is travelling. needs to be 25mm from electrical cables 140mm from sockets etc, if acs serves me right.

when you cook your veg where will the steam go? I dont think I would fit a boiler with a 10 year warranty over a cooker and expect it to be working in 10 years. So the answer is no.

You need to consult the boiler manufacture's installation manual, or phone them regarding the installation of another form of heat source fitted in close proximity to your boiler. Some times common sense must prevail, if the new appliance gives off a great deal of heat externally then you must make a sensible decision based on that!