The Gatestone Institute (formerly Stonegate Institute and Hudson New York) is a right-wing anti-Muslim think tank with a focus on Islam and the Middle East. The organization has attracted attention for publishing false articles and being a source of viral falsehoods
First sentence of the Wikipedia entry on the Gatestone Institute, further detail given if you go on the site.
We can now assess the accuracy and reliability of any statistics they publish. I am not sure I would place much reliance in them.
I suggest you look at the much more reliable figures produced by the House of Commons Library researchbriefings.parliament.uk/ResearchBriefing/Summary/SN04334 and in that document click on the download: Commons Briefing papers SN04334, Author: Georgina Sturge. This shows the UK prison population at roughly 83,000 (as others have said). 13,000, about 15% claim to be of the muslim faith. Most of those are guilty of all the ordinary every day offences of violence against the person, theft, sexual offences, robbery and drugs.
The number of terrorists among those is only 228. I quote from another reliable source Islamic extremists make up the majority of the 228 people imprisoned in the UK for terrorist offences, at 82 % of the total, followed by 13% far right, and 6% driven by other ideologies, including those linked to Northern Ireland. www.independent.co.uk/news/uk/crime/uk-prison-far-right-arrests-terrorists-conviction-national-action-a8398146.html.
There are several thousand people under investigation and many plots are spiked every year, but even then the numbers are very small.