That's a fair point for this year. However, it doesn't excuse the other years at other sites :-(

The height of the 'weeds' in your image does show that the field hasn't been grazed for quite some time, probably/possibly due to waterlogging - it is a lakeside field, afterall. If it had been grazed then the brambles may have been kept in check a bit more.

One a site is cleared, sheep or goats would do the rest. Unfortunately, in order to protect these sites they are often fenced off. It's a bit ironic really that by proctecting them in this way more damage could be done.