The Netgear D6300 is already an 802.11ac device so bettering that might be an expensive trial and error process.
Asus has a good reputation for wireless so the Asus RT-AC68U Wireless Broadband Router
at £160 is one to consider.
There is the AC87U
model that is in theory even faster.
A big issue with wireless is that while external antenna may allow you to move them around and get optimum coverage, if the building is attenuating the signal then almost no matter what router you will have problems and the solution is to add a wireless access point to improve wireless coverage.
Also any device that needs high sustainable speeds should be using 5GHz or Ethernet, since congestion on 2.4GHz is such now that it can be unreliable unless you can get neighbours to turn off their devices too.