There is no real need for rear speakers at all TBH.
I used to be seriously into car audio and a friend won the UK newcomer award with a simple setup of component fronts and rear sub. The positioning of the fronts becomes a little more critical but actually the sound staging of a simple pair of stereo is quite a clear refreshing sound. It will be lousy for rear passengers though, so if you have rear passengers maybe discard that approach.
OEM speakers are horrible in just about every car, they are literally £1 items.
Replacing a head unit is normally a 5 minute job, although a few cars takes longer as you have to get behind other parts of the dash.
Its worth getting a view on how good the existing head unit actually is, you could then spend the budget on a decent 4 channel amp, and all the wiring etc, along with a pair of nice components for the front, and a sub for the back. Run 2 channels for the front and a bridged pair for sub.
For most peope the biggest issue is the initial setup, finding where to route the cables etc, eg you need a fairly decent EARTHED power cable running to the amp(s), a cable from the head unit to the amp(s) along with some RCAs (probably). Then speakers from the amp(s) to the speakers. Thats quite a rewire job for most people and they get nervous with the power cable.
My first change was a head unit and rear 6x9s, I did this myself and it took a whole day getting the old head unit out, routing the cables etc. I then went up and had a simple upgrade with amp done for me and that took about 5 hours by a specialist.
Later I became friends with the guy who did the install and my car basically became a demo vehicle, test bench, but nothing touched the time taken to install decent cables well from scratch over the car.
Best day was installing 8", 4" and tweater components into custom door builds in the front. No one ever got into my car without going wow at the size of the front speakers. Although I actualy got a better sound later on by switching to just 6.5s and tweaters in those same door builds.
I would second a specialist over halfords any day. Just go it give them an idea of budget and ask for suggestions.
People always talk down on active subs but for the space they take up can be quite effective, yes they are not going to be as good as stand alone amps and subs but for simplicity in limited space they are not bad.
If I had a half decent OE head unit (and most tbh arent that bad), on that budget it would be amp and the best pair of speakers I could afford for the front. Fade the rear out completely and use the high level input to the amp from the existing head unit. That way you have some decent speakers and a decent amp, you haven't bought lots of cheap components you have bought a couple of good ones. You can then add more as you decide (if you decide), such as a head unit.