If it's a Howdens carcase, then it was prebuilt. If you remove the upper support bar (very safe to do) and the oven goes in, and will still be under the worktop, then you should be fine.
You can screw the sides of the carcase into the adjoining units, and that'll give you back the necessary rigidity.
The hob would be supported adequately by the sides, so doesn't require any structural support at the front, as pointed out previously, the front bar on the carcase invariably gets mutilated in order to get the hob in.
But one thing to consider, is that if the oven vents out at the front, then most women get fed up cooking over a hot stove, and getting blasted by the oven whilst doing so. Although it's a common standard to put the hob above the over, there's a very good argument for keeping them apart.
