Hey :-) Unfortunately you may not even get a keyboard in your UKCAT! It is a rare occurrence but can happen due to the unstandardised nature of the test across different test centres... The best thing to do is to prepare for the worst (i.e. use mental maths as much as possible, use the calculator as a last resort for the harder calculations and be prepared to have to use a mouse to manually click in the numbers) and then hope for the best!
Good luck,
Alex, 5th year UCL medic
6med