600mm is the "normal" size so certainly go for that unless you can't find a good one for the money. You probably don't need two ovens, but a separate grill compartment is more convenient than having the grill in the main oven.
Presumably you already have an electric cooker outlet on a dedicated circuit with a fuse/mcb of 30A/32A or more (it will not run off a plug and socket)
I gather you want an all-electric, not a dual-fuel with a gas hob which many people like. You might find a scratched or dented one at a discount somewhere, or a "last year's model" marked down, so try to get to a few big showrooms. You can occasionally get a good bargain like that at John Lewis.
Looking at "Which" best buys, there are no electric Best Buys under £300 (there is a gas cooker).
The Flavel ML61CDW at £299.99 from Currys scores 63% which is reasonably good. it has a separate grill and a fan oven. There were no other low-cost ones with ceramic tops.
Neither of the ones you mention has a test report, but I think I have heard that Logik is not a good brand.
Beko vary and some of their products are very good value.