My opinions:
28i Juniors aren't very good. I'd recommend going for a Worcester CDi (if you want Worcester) unless it is a flat/rental property. Personally in that price range a Vaillant Pro 28 installed by a Vaillant Advance installer (5 year warranty instead of 2) is a much better buy.
Unless you live in a very large property I wouldn't worry to much about F+R temps. You can size the radiators yourself by googling 'radiator sizing' and measuring your rooms.
Worcester Accredited means absolutely precisely nothing. Just means he installs a fair few boilers. It is no indication of quality, and in most cases you are paying for that extra warranty indirectly. Just so you know.
i would not fit a vaillant myself, we all have our own ideas what makes a good boiler and manufacturer.
what makes a vaillant advanced installer better than a worcester accredited installer out of interest?