The SIM Card can only be used for data, not for calls. The 30 day period starts with first usage, so you can order it in advance.
With this SIM card you can NOT use the "personal Hotspot" functionality many phones offer.
In the UK, there are 4 network operators: EE, Vodafone, O2 and 3 (Three). These use the frequency bands common in Europe.
Prepaid is most commonly referred to as "pay as you go (PAYG)" in the UK.
With all Internet access in the UK, only adult content is displayed unless this is explicitly deactivated (with proof of age). However, quite common services often fall into this filter, which is why it is advisable to check the age at the time of purchase.