I am not 100% sure that anyone actually attempted to respond to the question "why". I believe the OP understands that s/he cannot enter the airspace, but doesn't understand why that is.
Just north of Hawarden, is Liverpool which is a busy airport with commercial airliners flying in/out. The zone you mention (2000-3500') is the Liverpool CTA which is class D airspace. The airspace is class D, so as to ensure the safe and expedient routing of commercial traffic in and out of Liverpool airport. (big planes don't like us little planes getting in their way). The reason why it is between 2000 and 3500 (and not from the surface) is due to the fact that the airways that the big planes use (usually much higher than we fly) need a way to descend safely to the airport level, without having to risk it with us little planes getting in the way without their knowledge. This is why there is classified airspace along those routes. Airports that serve commercial traffic generally have STARs (Standard Terminal Arrival Routes) and SIDs (Standard Instrument Departure) [generally] with classified airspace along its entire routing. You can look at the standard routes by going to:
NATS | AIS - Home and looking at the STARs and SIDs. This may help illustrate why the airspace has its particular shape. If you would like a graphical representation, look at Flight Radar, and see how the commercial planes fly in/out of the airport:
https://www.flightradar24.com/53.38,-2.82/10
As mentioned above however, you do not necessarily need to go around that airspace, and you can request to transit the airspace. This is done by calling on the appropriate frequency [119.850], and requesting the transit - however also as explained above, you may very well be turned away, and always should plan a way around the airspace.
I hope this helps!