That wasn't clear from the OP's post.... Thanks for pointing it out
I second this - got a Kamei 330l roofbox and aerodynamic bars and fitting kit for around £300. Bars are transferable to other cars with only different 'feet' required which are cheap to buy rather than a whole system. I had one of my bolts snap off in my car roof so they sent me another pack out free of charge.
Yes. Click around until you find bundle deals.
Aldi did roof bars recently so they might be in store. Still available online https://www.aldi.co.uk/auto-xs-roof-bar-set/p/084289211566000 Ignore the poor reviews - they are all from people who have closed roof rails on their car. If there is a gap between rails and car, these are great quality for the price. They are rebranded StreetWise bars.