The digital version of BBC1, sent via your set-top box, will be a true 16:9 picture (on the vast majority of programmes), and so it will fit correctly on your TV, assuming of course you have 16:9 selected in the settings.
The analogue version of BBC1, which is what you get via the by-pass, or via an aerial, does not show true widescreen (16:9) pictures. Instead, the BBC do one of two things. For things where the original aspect ratio is important, like films, they show it letterboxed, with large bars top and bottom. Your TV probably has a zoom or movie mode for this, which will make it fit your set correctly. The picture will suffer compared with the digital version however, due to the decreased vertical resolution.
For other programmes, the BBC output their analogue content in a 14:9 aspect ratio. This is half way between normal 4:3 and widescreen, designed to look 'not too bad' on either. If you display this picture with your TV in 'wide' mode, you will get small black bars top and bottom. You need to flick through the available video modes until you come across an option like 14:9 or, as on my Sony TV, Smart. The latter in particular makes the picture much better by using a combination of trimming a little off, and stretching what remains to fill a widescreen set.
(At this point many hardened A/V fans will be shaking their heads slowly, and muttering, but relax. It looks a lot better than it sounds...)
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