I would at the classifieds for speakers looking for a stereo pair and matching centre, if you do indeed need a centre. You can then get cheaper speakers for the surrounds as these don't really need to be model or range matched. I'm always dubious about buying electronics second hand and prefer to fall back on the safety of a good warranty.
This is probably as good as you'll get new for your budget but it does give you something to build on.
Bit too low budget as the situation in the market is horrible with av-receivers. They are robbing everyone atm. Even the low model Yamaha costs 300£ at Richer Sounds, earlier 150£. No competition so they can ask what they want.
Something like this would be decent 5.1 speaker package. 3 different finishes available:
The multi-Award-Winning Wharfedale Diamond 9.1 bookshelf speakers are a real classic in the world of bookshelf speakers.
That would sadly leave 150£ for the receiver and most stuff at ebay is overpriced old items. You need something like these below. If you can up your budget little i would make an offer for the RX-V485 as it`s the newest model of the bunch (2018):
Find many great new & used options and get the best deals for Yamaha RX-V485 5.1 channel AV receiver at the best online prices at eBay! Free delivery for many products!
If you have small room and fancy to wall mount speakers then the DX2 system is quite popular choice. I don´t believe it´s as good as the larger and older Diamond 9 range though, but which ever would work better for you.