I'm surprised that nobody's specifically mentioned the Moto phones. The Moto G7 Power is really good and within your budget at Argos right now - and is also available from giffgaff for a bargain £139!
The Moto G6 and G6 Play are pretty excellent too.
Xiaomi phones are very good quality. They have a very confusing line up of models, though. It's worth searching for Xiaomi on HotUKDeals to see what's available and what others are recommending.
Huawei are good, but their better quality phones might be out of your budget