Astra was the Beacon Cycle house brand, according to Sheldon. As @Blam and @Daniel R Hicks say, it's a mid-range 80s bike (that's a compliment)! The lugs, while nothing special, aren't drainpipe thick - this is a good thing. It was probably built well. Crankset may be Stronglight, and the derailleur and front mech are probably Sachs-Huret. Basic components that go on and on and on.
I can't tell you much more than that, I'm afraid. But it's in excellent condition, so it's a keeper. Since you don't want to spend lots of money on it, it makes an ideal candidate for practicing maintenance skills!
The tyres will be shot, so you should buy some new ones (nothing fancy). I can't tell from the pictures if they're 700c or 27", but the tyre size will be marked on the sidewall. If in doubt, just take one to your LBS.
Check the spokes by giving them a squeeze - if they feel a bit loose, you might invest in a spoke wrench (inexpensive) and give each one a quarter turn, but no more. The wheels might be a bit out of true, but if they're within 2 or 3 mm, just go with it.
The brakes will probably be very mushy and not return well when released. A new set of cables is highly recommended. If you're unsure how to fit them, see this post. Blast some oil into the pivots of the levers, too. Addendum: Fitting modern adjustable brake shoes, with modern pads, will really improve the braking performance! Those old pads will be dry and hard, and even when new would have been vastly inferior to today's compounds.
While you're at it, I would remove and dismantle the brake calipers. Clean them thoroughly, and lubricate any and all washers and pivot points, especially the point where the springs contact the arms (friction here can prevent old side-pull brakes from staying centred).
Don't be afraid to tear down the brakes; just be methodical and document each step, so you reassemble them correctly. When you put them back together, the nuts on the centre pivot should be pretty tight (to eliminate the arms wiggling around from front to back), but not so much as to cause the brakes to bind.
The headset might feel a bit gritty when you turn the handlebars. It may also be loose - if you squeeze the front brake and rock your weight backwards and forwards on the handlebars, you will feel the fork moving around in the headtube. If either of these symptoms are apparent, you might as well learn to service the headset!
Same goes for the hubs - if they feel gritty, or if they have any play, you might as well give them a new lease of life...
For about £30-£40, you should be able to buy a new pair of tyres and brake cables, a tube of grease, some new ball bearings, a pair of cone spanners, and a big fat headset wrench. With those few bike-specific tools, an adjustable spanner, and a few household essentials (rags), you'll be able to service it no problem.
Just get stuck in! You can't really go wrong with a sunny afternoon and a cold beer. Sheldon tells all!
Final word to the wise: As it was manufactured by Motobecane, some of the tube diameters and threads may be non-standard. Do not lose the seatpost!