combi boiler over cistern

The main considerations are ventilation and safety of wiring - unless your bathroom is big enough to have the boiler electrics completely out of reach of wet hands it wouldn't be approved. Over a toilette should not be a problem, but of course if you can fit it beside the toilette or to another wall it would be better. It is common to see boilers fitted beside kitchen walls cabinets so when they do the maintenance they have the base cabinets in the way and I never heard anyone complain about it, i have also seen boilers fitted into lofts. Your boiler fitter should explain to you the scenario where best to fit the boiler. Mine is in an air cabinet in the bathroom beside the shower, power taken from the ceiling, ventilation right, so up to regulations.

No total twaddle.

A Boiler in bathroom and toilets are classed in a zone 1 or 2 category you cannot fit an appliance in a position where u could touch the appliance and water at the same time due to health and safety