Half day during aricleship period

Yes, there is rule of 35 hours in a week and in case of increase, the firm have to pay according to normal wage rates for the extra hours in normal cases. But sadly these rules are on paper only and in reality most of articles works 8 or more hours daily as you said and they affraid to go against their principal due to various known reasons. And further, we all know what happens in September month.

Now in your case, you are working half day on Saturday and your firm is deducting payment for that day. Its all depend on your firm's policy whether they are giving you a full day attendance or a half day attandance. It will treat accordingly. 

(Its my opinion, let wait for other answers also. And do remember articleship period counts on day basis no one count it on hourly basis)