The San Remo[1] Conference was an international meeting of the post-World War I Allied Supreme Council, held in Sanremo, Italy, from 19 to 26 April 1920. It was attended by the four Principal Allied Powers of World War I who were represented by the prime ministers of Britain (David Lloyd George), France (Alexandre Millerand) and Italy (Francesco Nitti) and by Japan’s Ambassador K. Matsui.
It was agreed –
The High Contracting Parties agree to entrust, by application of the provisions of Article 22, the administration of Palestine, within such boundaries as may be determined by the Principal Allied Powers, to a Mandatory, to be selected by the said Powers. The Mandatory will be responsible for putting into effect the declaration originally made on November 8, 1917, by the British Government, and adopted by the other Allied Powers, in favour of the establishment in Palestine of a national home for the Jewish people, it being clearly understood that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine, or the rights and political status enjoyed by Jews in any other country.
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The International community gave Britain the Mandate under a pledge to fulfill the Balfour Declaration. When they failed to do so, when in fact they did everything in their power to stifle Jewish aspirations, the British presence in the Land of Israel became illegal in and of itself.
That illegal behavior by Britain is what the Jewish underground fought against.
The real problem in Palestine was that the British had made contradictory promises to the Jews and the Arabs. As now, there were pro-Jewish and pro-Arab wings in the British establishment, which is why Perez's statement that the British state is fundamentally anti-semitic is such utter nonsense. A fundementally anti-semitic Britain wouldn't have allowed any migration into Palestine at all. A fundamentally anti-semitic Britain wouldn't have allied with Israel against Egypt in 1956, or sold Centurion tanks to Israel (in spite of severe opposition from the Arab States), which was a major factor in Israel's very survival in '67 and '73.
Nations do things for many reasons, but principally they are motivated by their own internal concerns. Just because the British government does a deal with some Arab country, or condemns the bombing of Gaza, or decides to limit the number of Israeli students allowed into Britain, doesn't neccesarily mean they are anti-semitic. They might equally well be motivated by good business, humanitarian concern and a desire to be "seen" by the British electorate to be doing something about immigration. If you go down that route, you can hardly complain if an Arab sees the invasion of Iraq, the continued presence of western troops in Afghanistan, and the campaign to stop a mosque being built at "ground zero" as evidence of anti-moslem opinion.
Your contention that the Jewish underground were fighting against "illegal behavior by Britain" is a flimsy alibi, probably thought up as an explanation long after the event. They were fighting for Zionism, pure and simple, against anyone or anything that even seemed to be in the way. Their campaign was all about power, not legitimacy. These things always are.