hi the first thing is to do your theory test, dvla do a cd rom with test questions and hazard perception on it too. a good riding school should have these available to purchase. iam in the middle of doing my das at the moment, i found the theory test ok really. try to find a riding school with their OWN practise facility not one rented from the test centre as you will only have limited time to practise the manouvers, if they have their own practise area you can practise till your forearms burn from feathering the clutch lol.
i passed my mod 1 last thursday in the pissing down of rain, 1 minor for a little too slow on the swerve test. also a bike with abs may make you more confident. the test centre in enfield is grippy even when wet. the practise area i used was smaller and more slippy than the test centre but this was good as if it can be done in a smaller area it gives you more time with the nerves to do it at the test centre. the swerve is not as dangerous as you may hear, there is however an easier way of doing it which your instructor should show you.
you tube has plenty of videos on mod 1 tests to let you know what your letting yourself in for.you should find it easier to make speed on a 500 ish bike than a 125, but if your doing the 20mph through the top bend making the extra 12 mph shouldnt be a problem. one tip for the test is to just take each manoever at a time and a deep breath before shoulder checks and take off. nerves got a hold of me kept thinking i was going to put a foot down, but if you take your time and keep your head up especially on the slow stuff it should be a breeze. the u turn is also nice and flat at the test centre so if you get the opportunity to practise uphill or on camber then it should be a breeze.
the mod 2 is just a 30 min (i think) road ride, you will have to follow some road signs, and keep an eye out for particular traps the instructors will lead you to, but once again your instructor shoud make you aware of these before the test on the practise runs.
i paid £570 for a 3 day course with bike hire, £30 insurance excess. and just turn up to learn. find a school your comfortable with who have all the right facilities and enjoy the experience. my das course is with onestopinstruction, great guys. got mod 2 this friday morning, fingers crossed.
hope this helps and good luck.