Ok folks, this (usually extremely busy) section of motorway is getting dug up for the next 8 months, they’re putting in crawler lanes for us on the hills. Nice move government! faints in shock
However, while they do this, there’s cones, lowered speed limits and stacks upon stacks of speed cameras along the whole section. The cameras aren’t working yet, but they will be very soon. I’ve counted at least 6 covering that stretch, and they cover both directions too. So watch your speed, and watch out for queues.
There are 3 possible ways round that hill if there is a problem.
1: J19, take exit towards Portishead, at entrance to town, follow B road to Clevedon, rejoin motorway at J20. Suitable for vehicles up to 7.5t during the day, though larger rigids and smaller artics could pass easily in quiet periods.
2: j18, take A4 into Bristol, take A370 to Weston-Super-Mare, rejoin at J21. Suitable for anyone who doesn’t need a police ■■■■■■ 
3: J18 (or j1 M32) take A370, then ringroad about 1/2 mile on, and A38 towards Taunton (past Bristol Airport), Rejoin M5 at J22, hilly but still ok for anything normal.