This is pretty straightforward. Lots of people have similar setups over shorter distances to distribute broadband service between buildings or work sites. It would cost about £500 for reasonable equipment, provided two conditions are met:
(a) Clear lines of sight from A (origin) to B (intermediate repeater) and from B to C (end). This includes no trees or other obstructions within 10 metres of the direct line from point to point - read about the Fresnel zone for an explanation.
(b) Access to power at point B. 5 GHz wireless equipment need not use a lot of power but consumption at point B will be 10-15W.
For the link from A to B use Ubiquity PowerBeams at each end - either M5-400s or 5AC-400s (cost ~£90 per unit). The 5AC-400s will give more speed but the firmware on the M5-400s is more reliable. For the link from B to C use NanoStation M5s or NanoBeam 5AC-19s. It is best to keep the system all 5AC or all M5 because M5 units cannot reliably talk to 5AC units.
The advantage of the NanoStation is that it has a built-in switch with Power over Ethernet (PoE) pass-through. This means that the only equipment required at point B would be a PowerBeam and a NanoStation with a cable connection between the units carrying signal and PoE plus one cable from the site providing PoE. Any other combination would require a switch and multiple power/signal cables.
One small but crucial point to note. If you use PoE passthrough on a NanoStation be sure to use the 1A PoE injector. This is not the standard equipment for a NanoStation but is essential when you have to units powered off one PoE injector. All of these items operate at 24V so you must use Ubiquiti PoE kit, not other 802.3af/at stuff you might have around.
All of this is standard repeater design and you will find guidance on setup, etc on the Ubiquiti website - www.ubnt.com.
With this equipment you should be able to get speeds of 30-40 Mbps in each direction (not simultaneously)