The al-Qa'ida-inspired fundamentalist group wanted to send five mail bombs to various targets over the 2010 festive period and discussed launching a "Mumbai-style" atrocity.
A hand-written target list found at one of the defendant's homes listed the names and addresses of London Mayor Boris Johnson, two rabbis, the American Embassy and the Stock Exchange.
The police counter-terror operation which led to their arrests was the biggest of 2010.
A total of nine men admitted various terror offences at Woolwich Crown Court and will be sentenced next week.
"Lynchpin" Mohammed Chowdhury, 21, and his London accomplice Shah Rahman, 28, were followed by undercover detectives on November 28, 2010, observing Big Ben, Westminster Abbey, the London Eye and the Palace of Westminster.
The conspiracy also involved three defendants from Cardiff and four from Stoke. It was stopped by undercover anti-terror police before firm dates could be set for attacks.
The terrorists met through their membership of various hardcore Islamic groups and met at specially arranged meetings held in parks in a bid to make surveillance difficult.
The nine, all British nationals, were due to stand trial but changed their pleas at the last minute.
Chowdhury, of Stanliff House, Tower Hamlets, and Rahman, 28, of St Bernard's Road, Newham, admitted preparing for acts of terrorism by planning to plant an improvised explosive device (IED) in the toilets of the London Stock Exchange.
They admitted the crime after the judge, Mr Justice Wilkie, indicated Chowdhury would receive 18 and a half years and Rahman 17 years.
The duo will only serve about six years because five are served on licence, prisoners only serve half their term as standard and they have already been behind bars for more than 12 months.
Brothers Gurukanth Desai, 30, of Albert Street, and Abdul Miah, 25, of Ninian Park Road, both Cardiff, also admitted the same count.
Prosecutor Andrew Edis QC told a jury, which had been sworn in but was discharged after the pleas, that the four involved in the Stock Exchange plot had not intended to maim and kill.
He said: "Their intention was to cause terror and economic harm and disruption. But their chosen method meant there was a risk people would be maimed or killed."
Usman Khan, 20, of Persia Walk, Mohammed Shahjahan 27, of Burmarsh Walk, and Nazam Hussain, 26, of Grove Street, all Stoke, admitted engaging in the preparation of terrorism by attending operational meetings in Roath Park, Cardiff, on November 7 and in a Newport country park on December 12.
Omar Latif, 28, of Neville Street, Cardiff, admitted attending the meetings with the intention of assisting others to prepare or commit acts of terrorism.
The fourth Stoke defendant, Mohibur Rahman, admitted possessing an article for a terrorist purpose on December 20, 2010.
The 27-year-old, of North Road, admitted being in possession of two editions of the al-Qa'ida magazine Inspire for terrorist purposes.
The nine were not members of al-Qa'ida but were inspired by the terror network, specifically al-Qa'ida in the Arabian Peninsula (AQAP).
AQAP uses internet propaganda to inspire radicals to carry out attacks against the nations in which they live.
Though no firm dates were set and no bombs created, the defendants had the wherewithal to do it, it was alleged.
They possessed the al-Qa'ida magazine, which contained a feature: "Make A Bomb In The Kitchen Of Your Mom."
The threats were not just facing London.
The Stoke quartet talked about leaving homemade bombs in the toilets of their city's pubs and discussed travelling abroad for terror training.
Speaking after the case, Deputy Assistant Commissioner Stuart Osborne, senior national co-ordinator for counter-terrorism, said the convictions would go a long way towards protecting the public.
He said: "We welcome the guilty pleas entered by all nine defendants today, following what was the largest counter-terrorism operation of 2010."
PA
