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The deadly assault by extremists on Nairobi's Westgate shopping mall is prompting some to ask themselves the inevitable question: could such an attack happen in Britain?

In theory yes it could, because it would be impossible and impractical to give every crowded place in the country 100% protection around the clock.

But in Britain elements of the police, the security service and the military have spent much of the last five years refining their plans and preparing for any repetition of the Mumbai siege of 2008.

That attack, which lasted three days and killed 166 people, was a wake-up call for the counter-terrorism community around the world.

It got them wondering how they would respond if this happened in their own cities.

In Britain there was a sudden realisation that the Metropolitan Police mobile armed response units were woefully lacking in firepower.

In practice this meant that if they ever found themselves in a shootout with suicidal jihadists armed with automatic weapons, in a built-up area, they were likely to come off worse and would have to pull back and wait for military special forces to arrive from their barracks.

Since then there has been a series of secret exercises held all around the country, codenamed Wooden Pride, whereby police counter-terrorism and firearms units have worked closely with units of the SAS and SBS to fine tune how they would respond to such an attack as quickly and effectively as possible with minimal loss of life.

Police firearms capabilities have also been upgraded.

So could the same Somalia-based group, al-Shabab, that carried out the Nairobi attack repeat it in Britain?

Computer memory sticks recovered from a senior al-Shabab operative killed in Somalia in 2011 revealed ambitious plans to attack various targets in Britain.

But Whitehall officials said on Wednesday that these plans had been only an aspiration and that there had never been any workable plot by the group to attack Britain.

They said the automatic weapons, grenades and large quantities of ammunition used in the Nairobi attack would be harder to acquire here in Britain.

Moreover, their geographical home is East Africa, where they have been able to move across porous borders, and it is a rich resource of funding and the recruits and facilitators who would have helped lay the groundwork for the Nairobi attack.

Al-Shabab, although formally in alliance with al-Qaeda since 2012, has as its primary focus the expulsion of UN-backed African forces from Somalia and then the establishment there of a strict Islamic state.

In other words, it tends to have a regional agenda, rather than a global one.

It does have sympathisers and fundraisers here in Britain, but its violent actions are condemned by much of the Somali community.

As of today it's estimated that somewhere close to 50 UK-based jihadists are still in Somalia, training and fighting alongside al-Shabab.

Around half are believed to be of Somali origin and the remainder from South Asia and other backgrounds. The authorities remain concerned that some may eventually return to Britain with new-found terrorist skills.

In recent years Somalia had been eclipsed by Syria as the jihadist destination of choice and very few British-based recruits are returning here from Somalia.

Meanwhile, British counter-terrorism officials have been studying what happened in Nairobi to consider what lessons, if any, can be applied in the UK.

The Metropolitan Police says existing advice to large organisations may be updated and the 91Èȱ¬ Office says it will be up to local police forces to give advice, if they choose, to shopping centres in their area on how best to protect themselves.