If you want a 144Hz monitor, you'll have to up your budget. 1080 144Hz monitors are still around 200 quid, maybe a little less if you're lucky.
Honestly though, you're better off going 1440p 144Hz IMO since you can find 1440p 144Hz monitors under £250, like this;
Samsung's JG50/JG52 is another nice 1440p 144Hz option, was £230 recently but has gone up to £300+ where available, unfortunately.
Best option for £150 is this IMO;
For RAM, 8GB of DDR3 on the used market is 20-25 quid, so yeah no reason not to add another 8GB for 16GB total really. I'm assuming you have 2x4GB now, just buy another 2x4GB kit. When mixing RAM, you can have some issues so just keep that in mind. For RAM to work best together, it's best to have the same speed/latency/brand/etc, but not entirely necessary. I've built a system with 3 mismatched DIMMs, a 2GB, 4GB and 8GB DIMM and it worked fine(a friend was adamant on using all the RAM he had in his system I was building for some reason lol) but you could get unlucky and have some problems mixing RAM.
Those are you 240GB options, around 30 quid. Though I'd personally just spend the extra 20 quid or so for a 480GB variant of one of those SSD's instead. If you're really set on keeping price down though, those 240GB options are great.
Lastly, GPU, this will obviously depend on your budget. It's also going to depend on what monitor you end up getting, as well as what games you'll be playing. You might also need a PSU upgrade depending on which GPU you upgrade to, so we'll need to know the make and model of your PSU, not just wattage. Would be handy to know the make and model of your case also, if possible.