Reliability doesn't really come into it. Each and every airline can experience irrops through no fault of their own. What if another of those North Atlantic Storms comes barrelling into the UK on the day of travel? Your plans will be entirely shot to pieces.

If you really must travel on separate tickets (and I fail to see why you have to, Turkish Airlines also fly from Manchester - and so does Egyptair, who would get you to Cairo non-stop) then you should always build in a stopover that is proportional to your need to make the connection. If it is absolutely vital that you travel from Amsterdam to Istanbul on that flight, then you should plan on getting to Amsterdam the night before and spend a night in the city (and extend your trip by an extra day). If it is critical that you travel on the Amsterdam to Istanbul flight, then go for the 7 hour layover. If you are content to take the chance and possibly have to buy brand new tickets to Cairo if your connection at Amsterdam cannot be made, then go with the tighter connection.

Personally, when I travel on separate tickets, I always make sure I get to the origin of the "main" ticket the day/night before. If the outbound of the "main" ticket is missed, then that's it, you can kiss goodbye to your entire journey. On the way back I can tolerate a shorter connection - in your instance, missing the very last flight is not so critical because getting from AMS back to MAN/LPL would be less of an issue than trying to cobble together a new AMS-CAI-AMS in the event that your very first flight (LPL-AMS) was late/delayed/cancelled.

A "cheap" ticket from another country is only cheap if you can a) reliably make use of the ticket and b) do not incur additional expenses in availing of it [connecting flights, accommodation, extra time off work] that eat-up the "savings". Otherwise, pay for the ticket from your local airport - that way, you have a greater degree of coverage, at least given that all travel is on a single ticket.