The first thing to note is that this is essentially a religious discussion in that the simplicity of the arguments being presented is in direct opposition to the complexity of the issues. Also the idea that air travel is Evil has such a high Truthiness quotient in the UK right now that there's little chance of those who believe it being convinced otherwise. However...
It is worth remembering that the idea of planting trees as an antidote to global warming has been around for a while. When Kyoto was first being developed it was known as "carbon sequestration". Some of the major US energy companies (led by AES, as I recall) were very keen on the idea and offered to buy up vast tracts of the Amazon to protect it. This was solidly opposed by the environmentalists, partly for reasons I shall explain below, and partly because it was seen as mere conscience-salving, not a serious commitment to Doing Something. I find the fact that people in the UK are now being encouraged to do something that only a few years ago Americans were being castigated for wanting to do rather ironic.
The idea of just paying to offset flights is even more of a cop-out, because for many people (probably more so in the UK than the US) flying is a small part of their total carbon budget. Bashing people who fly is popular because it is seen as targeted at "rich people" rather than at "ordinary people like me". (It also means bashing Ryanair, which is very popular with anyone who has ever flown with them.) If we really want to be responsible for our own carbon emissions then we ought to pay attention to our entire carbon budget. That includes cars, electricity and gas usage and so on. Furthermore, we shouldn't just cover our own usage. We ought to be aware that a fair amount of carbon is emitted in our names by our governments. This is probably more so in the UK than the US because the level of public services is higher. If we want to do our bit (or if we want to be able to look down our noses at those who are not doing their bit) then we need to offset our entire carbon budget. Targeting international travel and doing nothing at home smacks strongly of tokenism.
Then there is the whole issue of tree planting. There are good reasons why environmentalists regard it with suspicion. To start with it depends what sort of trees you plant. A fast growth willow planation that is going to be cut down in five years is a very different prospect to a sequoia forest. You can also get into the whole mess of saying "yes, let's plant trees to make biodiesel," only to discover that someone in Malaysia cutting down mangrove forests to make way for your commercial plantation. There's even some scientific evidence to suggest that cutting down forests helps the planet keep cool, so planting new ones is not always entirely helpful. Of course a reputable carbon offsetting fund will keep a close eye on the reforestation projects that it sponsors, but a reputable fund should also be aware that reforestation is not a sufficient response because it doesn't change our current profligate attitude to energy use.
The alternative strategies for balancing your personal carbon budget parallel what Kyoto does. They involve reducing your personal energy use, and using energy from renewable sources rather than from fossil fuels. It is therefore arguable that a better use of monies paid in carbon offsetting is investment in renewable energy and energy efficiency technology. This has the advantage of producing a long term reduction in CO2 production, rather than just trying to mop up what we currently produce. And it has the effect of making such technology more economically competitive with fossil fuels, so it will encourage those who don't care (or don't believe in) global warming to help out as well.
Of course one of the reasons that tree-planting is the offsetting method of choice is that far too many environmentalists have thrown their lot in with the NIMBYs and claim that all renewable energy projects are bad. If "visual pollution" is more important to you than climate change then the logical conclusion is to invest in nuclear technology, because that has a relatively small visual impact and produces no carbon emissions.
Finally it is worth noting that the price of carbon implied by these offsetting schemes are well below the price of carbon on international energy markets. There are good reasons for this, but it is also true that right now we are cherry picking, particularly in the area of reforestation. The more people who get involved in offsetting, the harder it will become to make effective use of the money and the more expensive offsetting ought to become. With technology investment we have some hope that the technology will improve, so we'll start to get more carbon savings for our money as time goes on, thus offsetting the lack of easy projects, but there is little hope that we'll find much in the way of savings in methods of tree planting.
All in all, it is a complicated mess that is very hard to navigate and in which it is all to easy to point fingers at people.
This all makes sense to me. It's so nice to hear some sensible discussion.