It seems based on my readings (and I don't just mean Sharpe books, I swear!) that the Napoleonic period saw a not insubstantial increase in the number of professional officers within the British Army due to its expansion, by which I mean those who entered it as a career, hoping to advance on merit, rather than simply expectation of class, having bought a commission. After all, nothing like a war to create vacancies...
I know that the purchase of commissions remained for many decades further, but nevertheless, I can't help but assume that there would be some effect from this so I was wondering how strongly it was felt at the time, and also whether it was enduring beyond that period, or instead dissipated with post-war contraction?
I know that this is an incredibly large delay in answering and I'm actually away at the minute however if I was bothered by time I wouldn't have studied history.
There were two main methods of attaining a commission in the British army at the time.
Purchase, as you mention it was possible to purchase commissions in the Kings army. However in 1803 the Duke of York successfully implemented restrictions on purchasing promotions. Purchasing the rank of Captain required 2 years experience and Major and above at least 6 years. This was later increased to 3 years and 11 years respectively in 1809 . This prevented the incidents of boys purchasing the rank of Lieutenant Colonel in a matter of weeks.
Vacancy. Promotion by vacancy was when an officer was killed in action the most senior officer in the grade below him in the battalion moved up the rank to assume his position. So if a Major was killed the senior Captain was promoted leaving a vacant Captain rank so the Senior Lieutenant was promoted, followed by the most senior Ensign, freeing up a vacant space for a new Ensign in the Battalion. Note that this only applied to instances of death in combat, not due to disease or an officer selling his commission. This sometimes resulted in officer lacking the experience to receive a promotion by vacancy in which case an outsider normally bought in. It was also much more difficult and discouraged to sell commissions obtained by vacancy and is a substantial grey area.
It was also possible for an officer in the militia to receive a commission in the army if he persuaded a sufficient number of militiamen to sign on to the regular army.
Now throughout the period from 1660 to 1871 the predominance of the officer's purchasing their commissions is clear with two thirds of them being purchased. In times of peace it was almost the only way to achieve promotion. However during times of extensive war the demand for officers, to replace those taken as casualties or for newly raised battalions, soon outstripped supply by conventional means.
In the Napoleonic wars the officer class was drawn from a much wider social base than both before and after. With men 4/5th of commissions in the army in 1810 being achieved by means other than purchase. This did not mean that they inherently came from the lower classes as volunteers could sign up in the ranks on the condition that they could receive a commission if one became available through vacancy.
Ultimately as demand outstripped supply from the gentry the social variety of the officer class greatly increased, with commissions going to deserving NCO's for acts of gallantry and the like. This was certainly a noticeable change and could be regarded as a 'professionalisation' of the officer class with far more men being promoted on merit. The reforms introduced during the Napoleonic wars like the Duke of York's I mentioned above and previous reforms such as the restriction on the minimum age of an ensign's commission are definitely professionalisation of the officer class. However even after the abolition of purchasing commissions in the late 19th Century the officer ranks were still largely from the upper echelons of society. So whilst there were reforms regulating commissions, outside of wars there was little change in the composition of the officer class.