Part of the problem you will have researching British Memoirs is that the Italians generally fought on the east coast of Spain while the British efforts were generally restricted to Portugal, western and southern Spain. In 1812 and 1813 there were some British activity on the east coast of Spain but it was minor compared to what was being done by the Anglo-Allied Army under Wellington.

The link below is to a comprehensive listing of British memoirs, letters, diaries, and journals from the Napoleonic Wars. It is divided into over 25 categories, including infantry and cavalry regiments.

The following British regiments fought on the east coast: 1st Battalion 10th Foot, 1st and 2nd Battalions 27th Foot, 1st Battalion 58th Foot, 1st Battalion 81st Foot, 4th and 6th Battalions of the King's German Legion, and the light companies of the 3rd, 4th, 6th, 7th, and 8th Battalions of the King's German Legion, as well as the light companies of the De Roll and Dillon Regiments.

You might want to check the folloiwing:

Hering, John Frederick. Journal of an Officer in the King's German Legion Cambridge: Ken Trotman; 2000.

Notes: was a medical officer. Served in Hanover, the Baltic, and the Peninsula from 1809 - 1811, with the 1st KGL Hussars. Served in Sicily with the 7th KGL Battalion from 1811 until late 1815.

Maempel, Johann C. Adventures of a Young Rifleman, in the French and English Armies, during the War in Spain and Portugal, from 1806 to 1816. This available from Amazon.

Notes: Captured in1811. Enlisted in KGL in 1812.

Phillipe. A Conscript for the Empire: the Experiences of a Young German Conscript During the Napoleonic Wars. Leonaur, 2008. 180 pages.

Notes: was a conscript in the French Army and captured at Bailen; enlisted in the KGL and was a friend of Johann Maempel.

Good luck!

Bob