have 4G data plans for my backup internet connection

I'm looking forward to taking out some PAYG SIMs with A&A when your carrier covers Shrewsbury (if that's Three, they say by the end of 2015). I'm particularly interested in the excellent upload throughput capacity of LTE for peak load-balancing of self-hosted services (i.e. will only shift load to the more expensive cell network when necessary).
I'd like to know that you'll price LTE data cheaper than your current price, akin to how you price your fixed-line Internet access with different prices for 20CN, 21CN, FTTC, etc., and also that you price upload and download differently based on congestion too.
Your current tariffs are 2p/MB = 0.25p/Mb = 20£/GB = 2.5£/Gb, presumably for both up and down. At ~4Mb/s up, that's about ~1p/s = ~60p/min = ~36£/h. On LTE, I see more like 20Mb/s up which, under the same tariff, would cost ~5p/s = ~3£/min = ~180£/h.
I'm not saying that the price per second of transfer should be constant; I'm not saying that the price per data should be inversely proportional to the throughput capacities. I'm just saying that I'd like the prices between 3G, 4G, upload, and download to be _disconnected_ such that they can be based on congestion independently, and vary independently in the future.

I forgot to mention peak versus off-peak times. Basically, I prefer the sort of pricing at http://aa.net.uk/broadband-prices.html#broadband-linetypes where the various prices are detached and can change independently in the future. The only thing that that table doesn't show is that upload is gratis. I'm not expecting gratis upload on 4G, but I'd still like it to reflect supply and demand separately from download.