The reliable way to hire a tradesperson

I have bought a single built in oven that would have been relatively straight forward to install as it’s a plug in. However I have miscalculated the depth of the cavity being that it would need allowance for the plug and safety. It protrudes around 3cm, is there way around to either : - Refit the plug sockets (there are 2, one for the hobs and one for the oven) so that they’re recess / hardwired? - pack in the outer frame of the cavity so that the oven can be securely fitted? - send it back to the retailer? I am in a purpose built flat so had expected the sizes to be reasonably standard. What would be the best way forward to manage this and have it safety installed?

I have a question about the recess left by the old gas fire. The back boiler behind it has been decommissioned but is staying in place rather than being removed, and I’d like to turn the opening into a decorative feature rather than leaving it as a hole in the wall, since there’s no plan to ever have a fire there again. Will I need a wooden frame before putting plasterboard on?

Do I need planing permission to install a WC/Shower room in my conservatory? I know Building Regs apply. House is a 3 bed semi. Any other points I need to consider? The conservatory roof is the original one - has not been changed.