Not necessarily. I would imagine a lot will depend on what intelligence the relevant services are able to obtain about who is looking suspicious. Since 7/7 we have learned about one or two significant terrorist attacks that have been rumbled, for example the one involving liquids on planes. They're the big stuff but they still require planning, obtaining weapons, scoping locations, etc, just as a small attack would.
Whether the perps had English accents or not, one of them appeared to identify with somewhere else, if the quote on the BBC website is anything to go by:
The ITV film shows a black man, dressed in a grey hooded jacket, saying: "We must fight them as they fight us. An eye for an eye, a tooth for a tooth." He added: "I apologise that women have had to witness this today, but in our land our women have to see the same. You people will never be safe. Remove your government, they don't care about you."
Islamist terrorism is just as bad as IRA terrorism or any other excuse for terrorism and the perpetrators should be treated in exactly the same way as any other involved in terrorism. I grew up with the fear of IRA terrorism and so I am not remotely sympathetic to terrorists or their cause and don't really care what other people think of that.
I'm not sure it would take just as much planning though. If you believe in that sort of radical Islamic teaching, wholly, there is no planning involved in picking up a weapon, taking to the streets and murdering people. There is in most cases and that's why people are caught, because you get multiple guys with grand plans, but there doesn't need to be.
I know you could plan it, and this seems to be planned to an extent (soldier, and the fact the terrorists were content to leave everybody else at the scene unharmed), but it doesn't necessarily have to be. Nutters do it all the time, whether it's running about with a knife in Japan stabbing innocent people, or shooting up schools in America. If you had the motivations and were crazy in the head then it's a matter of picking up a large knife from the kitchen and going out on a rampage, or going out in your car and running someone down.
There's probably a tendency to think they're being rumbled because of great intelligence and that's something I've always argued against; it's always been my feeling that terrorist attacks are an absolute rarity because terrorists are an absolute rarity.