I did mine with the British Deer Society at Wadhurst Park, and can thoroughly recommend them.
They have recently dropped the price too, I think you find they are very competitive.
They also run the course over four days, which means you will need to take some holiday. When their course dates are announced they sell out rapidly, as the training is highly regarded.
Their manual and DVD are very well done. The manual is not just for DSC1, it is also a good resource for further study.
They are a charity for deer welfare, well deserving of support, not a for-profit private company, if that matters to you.
Some others offer a shorter course, but I don't see how it can be done as thoroughly, and I think the longer courses work out as better value for money.
Find out the ratio of students to instructors i.e. class size.
Some package up the multiple choice question bank together with answers into an electronic game, so you can memorise the answers. Personally I don't see this as a useful way of actually learning the basics.
If you want to see what the exam entails, all the (up to date) questions are freely available at
DMQ Setting the standard for wild deer management No need to pay.
Get hold of a BDS manual, or sign up for the course and its included. You will find that the answer to every question is in the manual. In working out the answers you will already have learned a lot, and be better prepared for the course, which would be rather intense if approached without preparation.
If you aren't already experienced then the deer recognition photo test, and the shooting test are aspects where there can be no substitute for study and practice, memory skills won't help.
DSC1 is a proper examination with pass fail criteria, not an attendance course, and on my course a few did not pass first time. You could argue that a provider who claimed 100% pass rates must be better, or you might wonder whether there might be a tendency for teaching to pass the test rather than teaching to instill knowledge.
If you do fail part of the exam you can re-sit it with any provider, you don't have to return to the original one. The BDS make time on the final day to get new exam papers and offer resits whilst others are waiting for the shooting test. Check whether your provider can do this.
If you will be using the estate rifle for the shooting test, find out what the cost will be.
Good luck.