hiya,
i hav a yemans teenager cham and im in the uk too so i know what you mean, i would defo get a higher wattage basking bulb and either move the branches a little further away inside so its not so close to the bulb or add some more in there further down so he can choose where he wants to sit to warm up and bask, i would also sugest that if you dont want to do that ,get a exo terra reflector and suspend it above the viv so its shining down,but you will have to play around to get the correct temps its important to keep him cooler at night so he sleeps properly so dont worry about this as they can tollerate low temps at night providing he can heat up in the day, but during the day an adult wants ambient air temp at around the 80f mark and hotter at the basking area,also a cooler end at the bottom so he can thermo regulate(control his body temps) babys cant tollerate as hot as adults so go for 75 ambient and and again drop the basking area (and put more branches so he can decide where he wants to sit ,as long as there is a temperature gradient he should be fine... its only problematic when your animal cant get awy from the heat that your asking for probs.and if its to cold al the time he will become sluggish and wont digest his food properly,so best go for a higher bulb, i get a selection just incase and keep them in my reptile draw. good luck dude!!.
thats just my thought
