need some advise on my grow box!

Hi there! This is my first post on this forum as im new so don't really know what I'm doing I have a small grow box made from a chest of draws so only around 3 feet tall and 50cm wide and about 80cm horizontal way. Inside I have a 250watt dual spectrum light (I think it hps) and a tt100 extractor fan in top corner and a desk fan on the floor on speed 1. This is my first grow so I dont want to go over the top so I got 1 plant at the mo it's white widow from the medical seed company she's a bit droopy at the moment and I thought it might be from over watering she's only 3 weeks frm seed and about 3 inches tall but im starting to think it might be because it's just to cold! I have the box in my garage and im in England and this time of year its quite cold so inside the box can get to about 25/26:( Celsius with light on and off it drops to around 14!!! But recently it's got.colder and now with light on its around 16/17 and doesn't go higher! Today I put a heater in the garage outside the.box as it's quite hot so im wondering if I could get any tips on how to keep temps up and how to recover from over watering because she's stopped growing for about 4 days now and I can't figure out why haha! Oh I forgot to say I got it in soil bat special .so it has nutes for 6 weeks I was told in the shop. And I've been keeping my water ph between 6.2 and 6.8. Is there anything im doing wrong? Thanks guys!!

Back in my early days I used a heating pad. Like for your back.
It would be best not to have the plant sit right on it. Don't fold it either. The best place would be under the bottom floor like radiant floor heat. when the wood warms your cabinet will be perfect and your plant will love you.
If you can't put it under the cab stand it against the back wall either way you have control with a heating pad to adjust the temp.

You may not have overwatered but just too cold on the roots.
I'm in very similar climate straight across the pond. hope this helps.

Hi thanks for the advice! I will find out where I can get one of those,I actually didn't think of that but sounds like a good idea! Il have to do that as I checked on her today and she seems to be growing slowly but the old leaves are still droopy and the new leaves seem to be more perky and straight! (good!) but the temperature is still low (around 15c) so I placed the heater in the box for the.last hour of the 18/6 cycle and it quickly.got to around 30c! So im thinking that's way too hot too quick! So im going to go with the heat pad like u said see how that goes. Thanks again!