I'm sure I'm reading that wrongly but to me it reads that you can fell enough trees to create a glade or a ride - is that correct?
The creation of a number of glades and rides is generally seen as a positive as it lets in light, encouraging biodiversity, light loving flora at the edges and a variety of habitats within the wood. Provided you stayed within the guidelines of 5 cu metre of felled timber in a quarter, you could progressively create rides or glades without applying for a licence, although it might still be advisable to just run it past an FC officer for their thoughts. If you wanted to do it all at once and exceed the 5 cu m / quarter in one go, you'd probably still have no problem getting permission if your plans seemed well thought through and appropriate. Rides, access and open areas are necessary for timber extraction and processing, so are a normal and positive feature of many woods.
what I don't understand is whether or not it's a requirement for all of the work to actually be completed by the expiry time of the plan
When I discussed thinning my oaks with the FC, I said one option was to do it at a very slow pace, maybe just a few a year to minimise disturbance, and as such it might take many years. The FC officer said they usually grant a felling license to cover an agreed time frame, ( 10 years as I recall ?), but if due to variable factors such as health/weather/finances/available time, etc, it takes longer, there would be no problem granting an automatic extension if the original felling proposals are adhered to. Main thing they are concerned with is that you've given thought to replacing the felled trees, either through restocking or natural regeneration. Felling large areas and replanting is a normal forestry activity, the FC and large Estates see woods more as a harvest or long term timber crop rather than most SWOG members view of them as a semi-permanent amenity.
Its all on here;
http://www.forestry.gov.uk/england-fellinglicences