UK - PAYG (Pay as you go) comparison

(TL;DR - scroll down for summary).

Most operators offer packages or "bundles", valid usually 1 month after activation. Typically for 10£ you can get 500MB - 1 GB data and 150 - 500 minutes.

Below is comparison fo how much data and minutes 10£ will get you at different operators (texts are often unlimited).

Operator	Data	Minutes	Extras
Vectone	3 GB	500	Int'l calls included
Tesco	5 GB	-
Lebara	1 GB	500
LycaMobile	1 GB	500
Giffgaff	1 GB	500
Vodafone	500 MB	150
ASDA	500 MB	500
EE	500 MB	150
O2	500 MB	100
Three	500 MB	100	Check "unlimited" offer

Surprisingly, the "big networks" (EE, Vodafone, O2) offer much less per your basic 10 quid than MVNOs. I can only assume, that customers get some additional services in exchange.

If your data consumption is larger, here is comparison of how much 2 or 3 GB plus some minutes will cost you.

Operator	2GB	3GB
Vectone		10 GBP
Giffgaff		12 GBP
Lebara	12 GBP	15 GBP
Lyca	12 GBP	15 GBP (6 GB)
Lyca (data only)	10 GBP	12 GBP
EE	15 GBP	25 GBP (4 GB)
ASDA	20 GBP
Vodafone	20 GBP
Three		15 GBP (unlimited)

Few operators even offer "unlimited" internet. They don't allow tethering to other devices, so sadly that won't be replacement for home WiFi.

I would advise to read the fine print for so called "fair use" clauses, that would restrict your data after some amount, which mean that offer is not truely unlimited.

Operator	Price	Included minutes
Three	15 GBP	300
Giffgaff	18 GBP (20GBP after 01.04.2015)	2000
Vectone	20 GBP	unlimited

Too-long-didn't-read summary:

From comparison above, looks like most profitable offer is Vectone's "All in one pocket saver" for everyday use, and Tesco for data only.

Unlimited offers sound nice, but won't work with PC and are IMHO too expensive.

Austria - Most affordable PAYG internet

Most affordable offer for short-term stay in Austria, with large amount of data has Vectone.
For 10€ buy "Alles in einem Sparpaket" (All-in-one bundle) http://www.vectonemobile.at/en/bundles/all-in-one-bundle.aspx.

It gives 3 GB data + 500 minutes to Austria and abroad. Valid for 1 month.

The disadvantage: configuration of Vectone is not automatic. You must create APN profile manually:

APN	mobile.barablu.com
Username	Vectone

Another thing: when I went abroad (it's enough when you drive through Germany near to Salzburg), Vectone switched to something called "Vectone World", and could not automatically switch back to Austrian Vectone.

Austria - Contract - unlimited mobile internet

While in Austria, I used internet from3 (Drei) - 3WebCube

With WebCube you get mobile hotspot, that you can use anywhere in Austria (convenient, if you plan to move). 

The offer has changed a bit since I bought it. Drei added limited offers (to 9 and 20 GB/month) and unlimited with various speeds. 

The cheapest really unlimited offer costs 20€/month. Additionaly, service charge of 20€/year and 49€ for device. Contract is 2 years.

Wireless Nomad

I am constantly moving between cities and countries. Working as software dev, sometimes all you need is Internet connection.

Wireless Nomad series is about best short- and long-term offers for mobile Internet (and sometimes calls too), that I could find.

Mobile offers change often, so everything is up to date only on time of writing.