There is no legal impediment to British withdrawal from the EU or to seeking to achieve a looser relationship with Brussels.

Renegotiating EU Membership
The draft EU Constitution, signed by the Heads of Government of all twenty-seven of the current EU member-states on 29th October 2004, is only the latest in a twenty-year series of major renegotiations of the EU treaties: those resulting in the Treaty of Nice (2001); the Treaty of Amsterdam; (1997); the Treaty of Maastricht (1992) and the Single European Act (1986).

Each renegotiation has radically altered the nature of the EU. There is no legal reason why future EU renegotiations should not provide some or all member-states with a looser relationship with "Brussels" and with each other than hitherto. Such difficulties that might be encountered are entirely of a political kind.

Flexibility is already built in to the existing EU treaties. Fewer than half (thirteen out of twenty-seven) of the current member-states have adopted its principal economic and political project, the "single" currency.

The EU has long-standing free trade arrangements with its near-neighbours in Europe: for example, Turkey (a member of the EU customs union), Switzerland, Iceland and Norway (not members of the EU customs union). Outside Europe, the EU has a free trade agreement with a NAFTA-member, Mexico, and, once current negotiations are completed, will have free trade agreements with approximately 90 other countries -- around half of all the countries in the world.

Thus, neither on the grounds of precedent, nor of EU or international law, is there any impediment to a country like the United Kingdom negotiating with the EU an arrangement which suits it best.

Leaving the EU altogether
Neither would there be any legal impediment to the UK leaving the EU altogether. So confirmed the British Government, in the House of Lords, on 8th February 2007, in the following written answer to a written question:

EU: Withdrawal
Lord Pearson of Rannoch asked Her Majesty's Government: Whether, under international treaties presently in force, the United Kingdom would have the legal power to withdraw from the European Union if Her Majesty's Government or Parliament so resolved. [HL1863]

The Parliamentary Under-Secretary of State, Foreign and Commonwealth Office (Lord Triesman): Parliament may amend or repeal any existing Act of Parliament, including the European Communities Act 1972. There is no formal procedure for withdrawal in the EU treaties, nor are there any provisions in the treaties or any other international obligations which affect the ultimate ability of the UK to withdraw from the EU.
www.publications.parliament.uk/pa/ld200607/ldhansrd/text/70208w0002.htm

This formal statement of the position is consistent with the understanding of the government of the day in 1975, when, in its referendum on continuing UK membership of the EU, withdrawal would have ensued had the result gone the other way.

Greenland, then a province of Denmark, withdrew from the EU in 1985. The Swiss Federal Government, in its Europe 2006 Report on relations between Switzerland and the EU, concluded that EU members had, and will continue to have, an unequivocal right to withdraw.

Finally, the draft EU Constitution, signed by the heads of government of all 27 current member-states of the EU on 29th October 2004, but not in force, provides that any member-state may withdraw:
Article I-60: Voluntary withdrawal from the Union:
Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements

The EU and its neighbours
The current EU treaties (at Article 300 TEC) and the draft Constitution (at Article I-57) require the EU to "develop a special relationship with neighbouring countries, aiming to establish an area of prosperity and good neighbourliness, founded on the values of the Union and characterised by close and peaceful relations based on co-operation…the Union may conclude specific agreements with the countries concerned..."

These provisions would apply to the EU's relations with the UK in the event that the latter withdrew from the EU altogether, placing the UK on a legal footing vis-a-vis the EU no-less-favourable than that of Switzerland or Norway.