There are many jobs that are willing to pay you 'under the table'.
and while the risk might not seem very bad, the repercussions are quite extreme in the UK.
if you are in the
UK visiting or under a visa where you are not allowed to work, you can be deported and be refused entry for the next 10 years.
They are definitely cracking down on illegal working, with the implementation of foreign national ID cards, which employers and banks are beginning to require.
you aren't allowed to volunteer with a registered charity or charitable trust for the same reason as above. a little stringent but... thats the way of it.
a cash in hand job is entirely upon your discretion. they are available, but its you that will be affected.