What about the AAISP data sim? www.aa.net.uk/telecoms-mobile-data.html

£2/month regardless then 2p/MB via the Three network.

It is more expensive than the Three 321 Payg sim, but they won't care what device you use the data from.

the tethering detection is mostly based on APN settings

if three DPI Thinks you're tethering you get a Netspeed of under 1mb/s but no block (depends on the load of the local mast and if your on PAYG or contract) this applies to mostly Any Unbranded phone with a Three sim in it (the tethering allowance will not move be it PAYG or contract)

if you have a Three Branded phone the phones are norm setup to have 2 APNs, 1 for mobile and 1 for USB/Hotspot tethering (instant can't tether page on PAYG unless you buy an add on If using a Three branded phone,also data used on the mobile phone should not use the Tethering data up as its on a separate APN on the phone)

o2 uses IP based tethering detection (so using torrent without a VPN active can set off the tethering detection as there could be Peers in the swarm that only a windows or games console would normally connect to, happens a lot on giffgaff users who uses O2 tethering detection)

In reply to a post by leexgx:

If you have a Three Branded phone the phones are norm setup to have 2 APNs, 1 for mobile and 1 for USB/Hotspot tethering (instant can't tether page on PAYG unless you buy an add on If using a Three branded phone,also data used on the mobile phone should not use the Tethering data up as its on a separate APN on the phone)

I have an unconnected Moto G 4G with a three PAYG SIM now (it started off with an O2 SIM, only one I could get quick) and the Moto G knows the two APNs - "tethering" separate to "browsing" I think they're labelled.

You may not.

It's in the Terms and Conditions.

In reply to a post by georgelnx:

On android they cant tell unless it's really heavy usage

Not correct. They can, and do. It has nothing to do with your usage.

In reply to a post by kev_445:

If they notice a browser which isn't for a phone, they'll block the internet access.

It's more sophisticated than that.