For your purposes, if you don't want to use it for an extended period, and you also want to be able to access British WW1 military records, the minimum subscription you will need, is...
1 month Premium Subscription. £ 12.95
http://www.ancestry.co.uk/cs/offers/subscribe?sub=1Click on the compare tabs to see what that particular subscription gives you access to.
Be VERY VERY warned,...read the subscription and payment terms very carefully, because in regards to your monthly subscription fee, it isn't a one off fee.
If you don't notify them in the prescribed way and in the prescribed time period that you wish to cancel your monthly subscription, your membership will remain in force, and you will be automatically re-billed for the following monthly period, and that will continue each month until you cancel your subscription.
Also be aware that the ancestry search engine is one of the most awkward, difficult to use, and counter intuitive pieces of junk on the internet, and also, the records on ancestry have been transcribed by volunteer transcribers, and their efforts haven't been properly checked for errors, of which there are many, so take nothing on that site at face value, especially the search results transcriptions, and check the original documents yourself.
Some local libraries and public record offices in the UK do offer access to the ancestry site via a facility called the ancestry library edition, ask them, you might need to take a memory stick to download any records, they may also print them out for you.
I don't know if such access will include access to military records, so obviously, just ask them.
They may also possibly offer access to findmypast which is an excellent and very easy to use site and it's serch engine is very good and is also easy to use, findmypast does have some military records but not much from WW1, most of what you want will only be on ancestry.
http://librarywales.org/your-library/http://www.mytimemachine.co.uk/archives.htm