"Once enforced, there will quite simply be no more significant powers left solely with the governments of the member states, and outside the orbit of the EU's formal institutions. Of course, member states may still pursue their individual foreign and defence policies, for example, but their activities will be increasingly circumscribed by the EU and their national sovereignty diminished."
If one tIhinks of sovereignty as the power to redistribute wealth and the power to use force, then Lea's opinion is misguided and unhelpful as it feeds the sensationalized news depicting the EU as something like "The Blob," which is slowly taking over the competencies of member states.
Her argument takes things like an EU Foreign Minister for its superficial face value and ignores the fact that any powers (of negotiation, mandate, etc) given to the EU Foreign Minister will require unanimity of the member states. Every EU member state must endorse the powers given to the EU Foreign Minister and their ability to veto policies that go against their national interest means that they retain complete sovereignty over foreign and defense policies, despite the creation of a new job title for someone at the EU.
At best, the EU Foreign Minister might have the power to persuade EU member states to soften hard lines, but in areas of member states' core interests this influence is unlikely to make a difference. For example, changes to the French line on the Common Agricultural Policy (CAP) has been more the result of structural changes to the domestic agricultural sector within France than persuasion by other countries. I wouldn't rule out the possibility that an EU Foreign Minister might soften Estonia's stance to condemn Russia for criticizing Estonia's removal of Russian statues, but I would be doubtful of an EU Foreign Minister's ability to persuade the UK to remove troops from Iraq, renegotiate military cooperation with the U.S., or to lift the EU's arms embargo on China. These issues remain within the core interest of the UK, and the UK's continued ability to veto anything that touches on these issues means that it has not lost its sovereignty, nor has the EU reform treaty changed that ability in any way.
The reform treaty does not make any significant changes to other core competencies of member states, like taxation and redistribution. These remain the sole competencies of the member states. I acknowledge that the single market and the Eurozone (which doesn't apply to the UK) puts constraints on the redistributive policy choices that states can pursue within the EU because of the free movement of goods, services, capital, and labor (i.e. high corporate taxes in one member state can lead to businesses fleeing to other member states with lower corporate taxes), but this occurs regardless of the reform treaty and is a consequence of EU membership in general. Aside from the minimum and maximum limits on value-added tax (VAT) that EU member states agreed to impose on themselves to prevent a race-to-the-bottom and race-to-the-top, the supranational EU institutions have no say over who gets taxed how much and where that money goes. This power remains in the hands of member states and remains a key source of member state sovereignty.
Where the EU is perhaps stepping on areas you would expect to remain the sole competency of member states in education (the attempt to put for an EU History book) and health policy, the reform treaty does not add or take away from this in any way. The most shocking creeping on the part of the EU, to me, is the rulings by the Commission regarding health policy and the free movement of goods, services, and people.
Two activities come to mind: 1) patients can seek treatment in any EU member state and their home national insurance must pay the cost and 2) the Commission is trying to harmonize standards the pharmaceutical market, because I'm not entirely sure that mutual recognition applies 100% here.
The implications of these activities on European welfare states could be enormous. But the EU reform treaty does not address this at all, because it changes voting weights and does not make any significant changes to enhance the Commission's competency over EU competition policy or member states' ability to appeal these policies. If anything, one could argue that French President Nicholas Sarkozy insisting that competition be taken out of the EU's objectives, may weaken the EU's ability to make rulings that would increase competition in the medical sector, although such weakening of the EU's competency over competition policy because of Sarkozy's stance is disputed by the Financial Times.
The EU reform treaty does not erode the UK's sovereignty any more than being a member of the EU single market already has done. If one takes sovereignty to mean the ability to use force either against one's own population or another's and the ability to redistribute wealth, then the UK has preserved it sovereignty entirely, with the caveat that the single market has limited some policy choices. Whether or not these choices are limited, the ability to take decisions in these areas remain in the hands of member states and "outside the orbit of the EU's formal institutions." The EU reform treaty has not transferred these competencies to the EU's institutions, nor is it likely that any future treaty will.