Lycamobile 100GB data, unlimited min/text, 100 International min, go roam, 30 days - £11.90 @ Lycamobile

Mobile contract providers tend to increase your monthly tariff by inflation - and exactly which measure depends on the network:

Three: a Fixed Annual Price Change, 4.5% this year.
O2: RPI plus 3.9%.
EE, Vodafone: Consumer Price Index plus 3.9%.

The date of the adjustment also varies by provider:

Three: April
O2: Announced in Feb, effective from April
EE: Announced in Jan, effective from March
Vodafone: Announced in Jan, effective from April

SIM only is just that, where you only pay for the minutes, texts, and data you use on your sim-card.

You will have to supply your own device i.e phone/tablet to insert the sim-card into, hence why it's called 'sim only'