Navian boiler is it any good or is a bacon or Worcester boiler better

I would always stick with the more common known brands either worcester bosch or vaillant which are usually the most popular however most other known brands are reliable nowadays, ideal, potterton, glowworm, baxi, alpha, all offer long warranties and good after sales service. most combi boilers will struggle with trying to running 2 showers at the same time, you are better off with a conventional boiler with an open vent or sealed system or better still a direct mains fed unvented cylinder.

Stick to the brands that are easier to locate parts for