What’s interesting is that the Institute of Terrestrial Ecology paper (1999 - https://bit.ly/2VBwmDZ) lists those figures for trees that are 8-12.6m apart, which is not even close to the spacing that trees are typically planted at, even when you account for thinning out in a managed forest.
The figures are also for the lifetime of the tree, so if one can only guarantee a tree for half of its life, the per-year carbon sequestration figures will be significantly lower.
I’m unsure how you’re offering to offset for £5/month right now when you haven’t planted anything yet though. Consider me pessimistic but it doesn’t seem like you can guarantee the tree lifetime required to offset at that price.
Honestly anyone who wants to offset would be best off trying to get involved with a local well established tree planting group as one person can easily plant a few dozen trees in a day and the most expensive part (in the UK especially) is the labour of actually planting the trees.