If you want to transit the TMA VFR, there are a few things to consider which will ease your passage.
Firstly, avoid the peak times, since within them the airspace will most likely be running at capacity and you might not get a clearance for safety reasons. Capacity traffic allows a controller to be able to cope with the level of aircraft within the airspace and handle unforeseen problems such as emergencies. As a VFR aircraft will not figure in the predicted traffic loads, you will be an extra and might not be accommodated as you might be the one which tips the scales and causes an overload. The peak times are 0700-1000 Local and 1600-2100 Local. They are not guaranteed however, and it might be busy outwith these periods. Similarly, it will sometimes be possible to get a clearance within the peak times. All of it is down to the situation existing when you make your call.
If you want a better chance of transit, do as others have suggested, and route around the Southern part via the area of the TRN VOR. If you try and fly up over Glasgow and Edinburgh you will start getting mixed up in arrival traffic, departure traffic, and holding traffic. This will possibly involve extensive vectors since they are the majority flow and you will be a minority.
If flying above 6000'ALT, then know the IFR waypoints within the TMA, as your clearance might involve routing via them. Controllers will use 5 letter designators or navigational aids, they won't use VRPs or geographical features above this altitude, since they are TMA controllers and not airfield ones. At or below 6000'ALT, the main part of the TMA is delegated to Prestwick, Glasgow, and Edinburgh Approach controllers. They will know where you are talking about if you use normal VFR navigation techniques and features.
Remember the Transition Altitude, which is 6000'ALT.
If you plan to cross an Airway somewhere on your route, make sure it is a Class D one. You won't get any clearance to cross Class A Airways when flying VFR. If you take the Eastern route suggested by some, you will have to bear this in mind and descend below the appropriate base level when you cross P600.
Good luck, and you might find the weather up here is better a lot more than you might think. Certainly the last few months have had extended periods of wall to wall sunshine
