alternatives or complements to nuclear
'What are these "economically and technologically better or equivalent solutions that would probably solve energy needs more effectively."?'
For the UK, proven renewable resources are mainly wind electricity load-balanced using uprated hydro. Most building heating and hot water through improved insulation, heat pumps and passive solar thermal.
Basically if you overprovision wind as the main electricity generation source by a factor of 2-3 , you can handle fluctuations by holding back water in high dams (much of the dam capacity needed already exists) for when not enough wind is blowing fast enough over a large enough area for this otherwise to be a problem. Getting rid of bank holidays and replacing these with wind holidays based on weather forcasting reduces the extent of overprovision or gas turbine backup needed (fueled using Fischer Tropsch synthetic hydrocarbons if you really need an abosolute zero carbon footprint and are prepared to pay a bit more to obtain this). This is all proven in the sense we can work out what this all costs and plan it starting now once we decide we can afford it.
Whether this is more "economical" than nuclear depends upon historical cost reduction with expansion, remembering that nuclear is later in its cost reduction exponential curve than wind (meaning nuclear won't get much cheaper with expansion but wind will). Another factor affecting computation of "economical" depends upon better understanding the subsidies provided to both renewables and to nuclear. Subsidy is currently provided to renewables through feed in tariffs and to nuclear through the taxpayer underwriting the accident insurance (e.g. the cost of having to evacuate the population at a radius of 20km around a nuclear plant ).
In relation to currently unproven resources, tidal has good potential but needs lengthy but relatively cheap research, as with solar voltaic - our poor UK sunshine requires research to improve this technology before it becomes cost effective in the UK compared to combined wind/hydro . Solar thermal sited in the empty deserts of North Africa and the Middle East also holds promise, but this requires high voltage continental DC transmission and greater political stability in the generating regions.
To avoid putting all our eggs into one basket what we are likely to see in practice is all of the above, including one final generation of new nuclear reactors probably adequate to maintain 15-20% of current UK elecricity demand for another 20-30 years, following a careful review of siting with respect to coastal and flooding risks. The UK is likely to experience minor earthquakes, flooding and tsunami events ( e.g. as with the which also risks catastrophic failure of both nuclear plants and hydro load balancing or water supply dams e.g. see http://en.wikipedia.org/wiki/Bristol_Channel_floods,_1607 ).
Even with the 80% C02 reduction in relation to UK economy by 2050 which the above combination of approaches could theoretically achieve, the cost of climate damage caused by C02 here is likely to be much greater than the costs all of the above would amount to. When deciding what is "economic" we should account climate damage as a subsidy and therefore part of the total cost of fossil fuel use.