To answer this rightful doubt, we have to consider that the original layout would still host races, so it couldn’t be drastically altered. Therefore kerbs, studied and placed to get cars go clockwise, should be at best redesigned to work both ways.
Same issues are findable regarding Armco crash barriers: put down to be 100% safe on the normal lap, a reversed layout would see drivers dangerously facing the safety crew gates all around the track.
But even assuming all those works could be done, Formula 1 has plenty of money after all, run-off areas and tech-pro barriers would require modifications as well.
In particular, Silverstone is an old-fashoned circuit, with unforgiving grass patches on the entry of many turns, preventing drivers from gaining cornering speed by going wide. This happens, for instance, approaching Copse, Stowe, or, with a strip of gravel, at Abbey. By reversing the lap, those would represent the corners’ exits, becoming death traps for drivers even slightly out of the racing line. Regarding (normal) Turn 1, a bump on the apex would dangerously upset the cars’ balance as well.
Moreover, run-off areas should be redesigned on ‘new’ fast corners, such as Club, Stowe and Chapel, and some of the grandstands would have to be replaced as well.
On a racing prospective, overtake opportunities would be reduced to the minimum, with the now wide approach to Luffield representing the only clear door openable for attacking cars.
End of the day, such a Mario Kart-ish solution would for sure be something to have a look at, even just for the sake of curiosity, as the race could go down both to mayhem or boredom. But we have to look at the real world as well: why would Formula 1 invest so much money in a one-off event, when tens of other circuits around the world would pay millions to welcome the Circus on their facilities? Would Formula 1 risk spectators’, track-side workers’ and drivers’ life by rushing the renovation? Bare in mind that local organizers have worked on Zandvoort’s restoration for almost a full year and it has been quick for the sport’s standards.
So, no, probably we are not going to witness a backwards race at Silverstone anytime soon and, after all, there are good chances we won’t be missing anything epic.