At least in my post code (not in NW though) their web page gives a reasonably good estimate of non-prime time speed. You might get up to 30Mbps there. I gave it up as it was useless in giving me a speed boost to achieve HD/4K streaming speeds. Generally, around 9PM, speed was not even close to the "up to" -speed, probably due to network saturation. I managed to get around 20Mbps at night but only 0.5-2Mbps evenings.
If it is only for a month or two, Relish can come a bit expensive as you need to pay the £50 router delivery charge + then monthly fees. They waive the router charge only for 12 month contracts. I still have my old Relish router in a pile somewhere as I had to buy it being unwilling to commit to 12 months.
If you can live 1-2 months without Netflix, just get a 4G dongle. It will be less than £90 for two months. And even if you cannot live without Netflix, Relish might not help you much unless you are watching TV in the 2AM-10AM slot. Then again, your area might be different. In my neck of woods 50% of us do not have anything better than 3Mbps ADSL lines, and quite a many probably use Relish. It might not be that common in your area.
The connection is not tied to your location. If you know anyone with Relish, borrow the router for a couple of hours and see how it works.