Everyone I know who's ever fitted their own kitchen has always said that it was much more troublesome than they'd expected and that they'd probably never do it again.
That said, they all finished without any major issues - just involved more time, effort and frustration than they'd anticipated.
In my old house I fitted my own kitchen. It was far easier than I expected and the end result is still good today (6 years later). B&Q units, worktop from worktop express, sink/oven etc sourced wherever was cheapest.
Measure until you are 1000% sure of your measurements. Start from the hardest corner and work out from there. Laser spirit level was a HUGE help for getting everything level and consistent
You will need to invest in some good tools if you dont have them already (drill, circular saw, jigsaw, spirit levels, laser level, poss router if joining worktops). Take your time and dont rush. Things WILL go wrong/not according to plan so be prepared for it to take longer than expected and cost more than expected.