In case you have to collect and check your bags. It's usually about a 5 minute walk along a corridor and down some stairs to border control where they now have passport scanner entry machines. If you are not from UK you need to look into the new entry scheme as you'll be headed into the UK for a few mins. You can collect luggage straight after. Then customs immediately after luggage carousels (I've never seen anyone actually operating customs) and another 2-3 mins to get back land-side. when you get land side keep going left down to the AF/KLM check-in area its on the left wall past TUI and EasyJet and before you get to a whole bunch of RyanAir desks, actually just in front of them . When you are done there head back where you came and you'll see some lifts on the left side, they take you up to security, there are some stairs but unless theres a huge queue by the lifts use the lifts the stairs aren't worth the bother. If security is quiet 5-10 mins and you'll be through. If its busy and looking like you might not make it go to the left side of the security gates and talk to someone there, they are pretty human (its not USA). 50:50 chance the machine wont like your hand luggage and it will need a manual check, add 5 mins. Another 5 mins to walk through duty free and then you'll be in a busy area with shops and stuff, just keep going straight. Go past a WHSmith on your left then turn left before "Shelby and Co" a bar. Up about 1 min turn left and another 2 mins to the gate, you'll likely be something like gate 42-46 if you are KLM. KLM usually go from the gates on the right side as you walk up. If you get delayed and you have priority pass (+£15) or are in J the Clubrooms Lounge is the one to go in. Its right near those gates. I think in the last few weeks there might be a new Indian Restaurant opposite now. Adding those time up if you have an easy time at border control - Ive never really seen it busy - I think its doable in 30 mins unless you hit a snag

I've never transited just been in and out but as others said you are going to have to go land-side and back through check-in and then through security.

TLDR You'll be OK but the above might help you navigate,

Good Luck