You do not need to spend thousands on a bike for the sort of riding that you do. A good entry level machine will do you fine.
There are two main issues when looking at a bike. Weight and reliability. The heavier it is, the more weight that you have to move. Many cheep components will wear quicker and will not be able to maintain reliable shifting etc.
You can ride a male bike, but bear in mind that your legs are longer than a males with the same body length. This means that female specific bikes have a shorter top tube, so women riding male specific bikes that are the right height for them can often feel stretched out over the bars.
However, A good starter bike would be the Specialized Hardrock Sport. About £2-300 depending upon the model. Not a bad machine, with reasonable kit. A good tool for what you want.
Scott are also a good brand, but you will pay more for their Contessa range. That said, Scott put a lot of work into making their frames light. For these bikes you are looking at about £400.
Many of the main manufacturers do a female specific range. However, you get what you pay for. Do not look at full suss and Discs for less than about £800. Boardman do a good entry susser at this price. A decent hardtail, with discs will cost about £5-600.
So for the sort of riding you want, you do not need a lot of travel on your forks. You don't really need the extra weight and hassle of discs. Go for a simple, honest tool, with 21 or 24 gears. Go for pods, rather than shifters, as this is easier on your hands, allows you to customise your grips for Eurgonomics if you need, and is easier to maintian.
Look for branded components, like Shimano and SRAM on gears. Make sure that this is supplied on the shifters and derailiers. A common trick is to put a brand name on thee rear derailier, where it is visible and then budget stuff where it actually matters.
Sizing your bike. Go to the bike shop, stand over the machine. You should have between 2 -4" clearance from your girly bits and the top tube. Anything else gets painful when you dismount (Feet 1" apart while standng over bike). Adjust your seat post so that your leg is fully extended when you peddle with your heal. This means that your leg is slightly bent when peddling. Reach over the bike and see if this is compfy. Take it for a spin in the car park.
If your saddle is quite low and close to the frame with your leg extended. The frame is too big. If you do not have about 1 1/2" of seatpost in the frame, it is too small.
Accessories:
Lid
Mini pump
Gloves and padded shorts/tights.
Multi tool (Parktool do a nice one)
Chain tool if not included in multi tool
SRAM power links.
Spare tube.
Puncture repair kit.
Tyre levers.
Lights if riding at night.
Cateye do some nice rear lights. You might also look at a set of Electron Halogens for about £40.
Go to these to see whats out there:
www.chainreactioncycles.co.uk
www.wiggle.co.uk
www.rutlandcycles.co.uk
luck