I didn't know about this method but I would assume you would use Ukash credit to fund your cashU account then credit MtGox. It's a really dire situation for people to buy small amounts of bitcoin cheaply in the UK. GBP/BTC exchange volume used to be the second biggest market when there was Intersango and MtGox Barclay's deposits.
I received a £100 Ukash voucher last week. I converted it to USD and then tried to deposit in cashu. The transaction failed and the voucher was marked invalid. I had to contact Ukash by email. They told me that cashu had stopped accepting ukash. I had to get them to deconvert the voucher back to GBP and I sold the voucher to someone directly for BTC. It took 2 days for Ukash to sort out the mess.