Under current UK Law sheriffs are appointed officials of the crown. Was this always the norm?

It is important to note that there are three separate jurisdictions in the UK: England and Wales, Northern Ireland, and Scotland; therefore, (arguably) there is no such thing as "UK law".

Sheriffs (or high sheriffs) are appointed officials of the Crown in England and Wales and Norther Ireland only.

The king's gerefa or reeve was an important official (after the alderman) and presided over the gemot enforcing folkright. It is difficult to date when the king's reeve was first put in charge of a shire, but the earliest recorded case of a reeve holding a shiremote seems to occur around the second half of the 10th century.

As Morris puts it:

It may therefore be inferred that the office of sheriff originated in the half-century between the enactment of the laws of Edward the Elder and the death of Edgar.

Morris links the rise of the sheriff to the centralization of local government. While it is generally accepted that the king appointed the sheriff (not necessarily for life), his duties were numerous and he was also an agent of the earl as well:

In a very real sense he was at once the officer of the earl, the king, and the nation.

Election was very much the exception not the norm. London is probably the best known example. The sheriffs of the City of London are still elected. Morris mentions that in 1225 (and again in 1230) Devon paid 200 merks for the privilege of electing their own sheriff to serve for three years. Dorset and Somerset also paid a 200 merks fine for electing Roger de la Ford in 1222. The men of Cornwall also claimed the privilege to choose their own sheriff. In 1300 Edward I allowed certain sheriffs to be elected, although apparently there was only one shrievalty in which election was adopted. In 1301 (or 1302) Richard de Harley was elected as a sheriff in Shropshire and Staffordshire. He was found to be unqualified for the role and a new election was ordered. The same person was elected, but protest were made on the grounds that certain barons, knights and abbots had not consented. Even though his election was formally certified by six knights, the lack of support from these men was more important and the king's council declined to uphold the election. A knight, Walter de Baysi, was appointed to the office.

Sources:

William Morris, 'The Office of Sheriff in the Anglo-Saxon Period' (1916) 31 EHR 20

William Morris, The medieval English sheriff to 1300 (MUP 1927) (freely accessible)

(Note that these are relatively old sources, so it is possible that scholarly discussion has moved on.)

In Scotland sheriffs were probably introduced by David I (1084 – 1153) and the office was judicial, financial, administrative and military in its nature. Sheriffs dealt with (and indeed continue to deal with) civil and criminal matters, but the country was covered by a number of overlapping jurisdictions. Sheriff courts operated alongside regality courts, burgh courts, barony courts, stewartry courts, constabulary courts, commissary courts and bailiary courts. From 1532 sheriff courts lost some of their jurisdiction to the Court of Session (e.g. issues relating to fee and heritage). Sheriffs in Scotland were never elected, in fact, the office of sheriff became hereditary and remained so until 25 March 1748, when heritable sheriffships were abolished by the Heritable Jurisdictions (Scotland) Act 1746.

Sources:

Stair Memorial Encyclopaedia, Courts and Competency, paras 139-150

William Croft Dickinson (ed) Sheriff Court Book of Fife, 1515-1522 (Scottish History Society 1928) (freely accessible)

(I am more familiar with Scottish legal history so you may wish to consult the sources linked above.)