BeoLab 5s FAR more than match their price tag. Have a listen to them and you may well find the price tag justified! Either way, if you would prefer, BeoLab 3s are very ideal as rear speakers as you can get them wall mounted high up (which the rear speakers almost always should be).
Either way, bear in mind that sound is half the picture and it therefore logically follows that if you're spending nearly £20k on the BeoVision 4 (with BeoSystem 3), you should look to spend near that on the speakers.
As far as the centre channel goes, you have 3 options, the horizontal speaker (which is available in two sizes), a vertical speaker and a 'triangle' speaker. The latter two would provide a better sound when used with BeoLab 5s, so I would personally opt for them instead.
If you don't want to spend the money on the BeoLab 5s, I would recommend BeoLab 9s with a BeoLab 2 (although the BeoLab 2 is only necessary if your room is very large). Bear in mind too, BeoLab 9s + a BeoLab 2 isn't that far off the price of a pair of BeoLab 5s, but the BeoLab 5s will offer a much higher standard of performance.