Three SIM Only ADVANCED plan includes 'Feel At Home' (free roaming in 42 destinations abroad), ability to create a Personal Hotspot (a.k.a. Tethering) and Free calls to Customer Services.

*UPDATE: alternatively @kris1234 was able to get 8GB on an ADVANCED plan with 600 minutes (and assumably unlimited texts) also for £10 per month*

This deal is based on previous HUD posts advertising Three's SIM Only 12 month ESSENTIAL Plan for £9 per month (total cost = £108) which may be found here:
http://www.hotukdeals.com/deals/3-sim-4gb-data-unltd-mins-unltd-texts-9p-m-108-term-possible-quidco-2505078
and:
http://www.hotukdeals.com/deals/unlimted-minutes-texts-4gb-4g-data-12-months-contract-from-3g-via-uswitch-9-month-2540875

A few HUD users (particularly on the 2nd link above) were commenting that they were able to get upgraded from the ESSENTIAL plan to the ADVANCED plan for an extra £1 per month (ie £10 per month total) if they were existing Three customers, usually achieved via live chat haggling.

I am not an existing customer so tried several times on live chat to see if they would offer new customers the £10 ADVANCED deal but to no avail, until finally one chat advisor suggested I started a 1 month contract and then tried to upgrade.

So I stopped by a Three store today to sign up for the cheapest 1 month contract I could find (which happened to be the ESSENTIAL plan with 500MB Data / 200 Minutes / All-You-Can-Eat Texts for £9 per month) and picked up a free sim card (and a free nano/micro/standard sim adapter...which was nice!).

When I got home I immediately went to live chat and politely asked to be upgraded from the £9/month ESSENTIAL plan to the £10/month ADVANCED to which the reply was 'No worries at all :smiley:' and told it will be ready to use from 5 days time.

I've checked in 'My 3' online and my price plan has been updated to:

4GB data.
All-you-can-eat minutes.
All-you-can-eat texts.

12 month contract.
Included:
4GB Personal Hotspot, Feel At Home

The same plan is normally £21 per month (total saving per year = £252 - £120 = £132)

Win! :smiley:

Important notes:
1. The deal includes a £5 monthly discount for paying by direct debit so, obviously, if you aren't paying by direct debit then add £5 per month.
2. The ESSENTIAL plan deal (4GB for the price of 1GB) that this upgraded ADVANCED plan is based on expires tomorrow (10 Nov) night at midnight (see the countdown timer on the http://www.three.co.uk homepage). I don't know if that will affect the validity of this deal but I have expired the HUD deal at that time just in case.
3. I didn't bother trying for any further offers (eg Amazon refer-a-friend gift cards, Quidco, Topcashback etc) because all my dealings were direct (either face to face in store or via live chat) but if you want to give it a go then by all means see the links (and the deal comments) at the top of this post.

Some useful links:
http://www.three.co.uk/Store/SIM/Plans_for_phones Link for choosing initial 1 month contract (or sign up in store like I did) - link also available via deal URL at the top right of this deal page
http://www.three.co.uk/Contact-us/i Live Chat
http://www.three.co.uk/store/compare-phone-plans ESSENTIAL vs ADVANCED plans
http://www.three.co.uk/Discover/Phones/Feel_At_Home Feel at Home info
http://support.three.co.uk/SRVS/CGI-BIN/WEBISAPI.DLL?Command=New,Kb=Mobile,Ts=Mobile,T=Article,varset_cat=internetapps,varset_subcat=3582,Case=obj(36401) Personal Hotspot info