If you can stretch your budget you could get the huawei p8 lite (2017)
link. It's currently £150 + £10 topup (
link) + you probably have to get it unlocked via buying an unlock code on ebay.
Samsung galaxy j3 2016
vs huawei p8 lite 2017: (specs wars)
CPU (max) 1.5 GHz
vs 2.1 GHz
Display resolution 1280*720
vs 1920*1080
Display size 5.0 inches
vs 5.2 inches
1.5GB ram
vs 3GB ram
8MP (rear), 5MP (front)
vs 12MP (rear),
8MP (front)
Battery size 2600mAh
vs 3000mAh (the battery is bigger on the huawei but so is the screen and resolution and so it needs a bigger battery)
Sensors: Proximity, ambient light and accelerometer
vs Fingerprint, ambient light, accelerometer, proximity,
compass
Storage 8GB
vs 16GB
Both phones have micro SD. The huawei is also on android Nougat.
I don't know if the significantly better specs are worth it to you but imho the p8 lite 2017 is the best value smartphone right now in terms of where price meets performance. Any cheaper and the spec's get a lot worse and if you want significantly better specs it's going to cost quite a bit more (since the lenovo p2 on 3 network is sold out AFAIK). Having said that you could get a better spec'd phone if you are willing to buy it from china (
link for info). Pro's: 3GB, SD625 CPU (it's about the same performance as the one in the p8 lite 2017),
32GB storage, huge
4100mAh battery. Cons: you have to select slow delivery to avoid uk customs (but still no guarantees) and it might take up to 20 days to arrive, warranty would require sending it back to china and hoping nothing goes wrong. It also has a 5.5" screen size, is that too big?