Alex did not cheat for the following reasons:-
1.If he had agreed with Tim to pick him, then why did he pick PJ first? Surely that was dangerous as Jonny was most likely to pick people who had been on the rich side before. Alex had two chances to pick Tim, but used neither.
2.Alex was clearly shocked when he came out of the bedroom & was told for the second week running that he would have to pick people. He assumed that the housemates decided amongst themselves who went where
3.If Alex did not know he had to pick, perhaps Tim has twigged & tried to make a pact with him, unbeknowst to Alex what they were shaking hands on.
In that case it was Tim who actually attemted to cheat, not Alex