I was having a look at the Appendices to Ofgem's Grid Capacity Assessment for 2014 (as you do) and chanced upon the section on interconnections. Now obviously, if you are going to make use of interconnections to other countries, there needs to be surplus capacity at the other end of the cable. It's therefore interesting to see Ofgem's assessment of grid capacity in countries from which the UK can import power.
Ireland
The transmission system operators’ (TSOs’) Base Case assumes full exports from GB to Ireland through the East-West and Moyle interconnectors, including at peak times of electricity demand in GB. ... The TSOs have also assessed a number of sensitivities, for example, the impact of the loss of interconnection with GB. In this sensitivity, Northern Ireland would face a capacity deficit in 2016 if no interconnector flows from GB were available.
France
In its 2013 report, the French TSO expects no capacity deficit (relative to their LOLE target), however this conclusion includes the assumption of around 7 GW of imports. ... If imports from neighbouring markets are unavailable, France would face a capacity deficit of 6.5 GW and 7.5 GW in 2016 and 2017 respectively.
Belgium
In any of the [its] scenarios, Belgium would be dependent on electricity imports to meet domestic demand. It has been assumed that the two nuclear reactors that faced problems in 2012/1317 are available in all subsequent winters. Were these stations to experience any further issues, the generation adequacy situation would come under increasing stress in the period 2014-2016.
[This seems to predate more recent problems with nuclear capacity in Belgium. Note also that Belgium is not directly connected to the UK, but is connected to the Netherlands, so may affect the UK market.]
Germany and the Netherlands
Both seem to have adequate capacity
You could get the impression that the whole thing is just a house of cards, with everyone expecting to import non-existent surplus energy from everyone else. I can't imagine that grid operators are not aware of this, but I wonder if there is anything they can do about it.
We currently import a fairly steady 2GW from France and 1GW from the Netherlands.