The pound coin as a unit of everyday currency is a pretty modern invention: it entered circulation as recently as 1984, replacing the £1 note which had previously been used in various designs from 1797 onwards. There were a few 17th Century attempts at creating a "£1" coin, but none that particularly took off. (EDIT:' thanks to /u/cnzmur for reminding me of the sovereign, which entered circulation in 1816 and was withdrawn in Britain in 1914, although survived longer in the colonies.)

One of the major headaches with English numismatics is the terminology and the uses of different metals. A lot of discussion of coinage and its precious metal content is actually carried out in Troy grain weight, but I'll convert (roughly) to grams as much as I can for ease. Early Medieval English coinage, which would form the basic template for all English currency throughout the Medieval and Early Modern period - and indeed technically right up until 1971 - was based chiefly around the silver penny. There were 12 pence in a shilling (4 in Mercia), 20 shillings in a pound, and so 240 pennies in a pound. Early Medieval English coinage is remarkably consistent in its quality and content across the period. Extant coins - of which we have an extensive corpus - average out incredibly roughly - at between around 1.3g to 1.5g in weight, accounting of course for a wide array of erosion, wear, shattering, chipping and other archaeological damage, with intact coinage weighing in on average around 1.5g - 1.6g. We also have extant a number of coin weights which seem to have been specifically designed to weigh out values of coinage; WINCM:ARCH 4893 appears to have been a two shilling weight, and the extant artefact weights 33.12g. As Gareth Williams states: "this marginal discrepancy seems perfectly acceptable, since the piece, although the surface is slightly worn and battered, is substantially intact and well-preserved, and therefore seems likely to have lost only a little of its original weight." It's worth noting here that the shilling is a unit of value rather than of commerce here: a shilling is the value of 12d (4d in Mercia) rather than an actual coin with the value of 1/20th of a pound. "1 shilling" would be paid with 12 silver pence. This would largely be the case throughout much of the medieval period: while there were a series of numismatic reforms, these largely introduced variations to the penny such as the long cross which curtailed fraud or aided small value transactions.

Higher value currency for everyday transactions finally entered mainstream use largely in the 14th Century. One prime example of this is the groat, a 4d coin which first appeared in England during the reign of Edward I in the late 1200s. Technically speaking the groat should have weighed 6.2g, being equal to the weight of 4 pennies, but the initial issue was a little light, coming in just under at about 5.8g, with successive issues getting lighter.

What this all means is that, broadly speaking throughout much of the medieval period, you could very roughly determine the value of a purse of coinage by simply weighing it, as the majority of the coinage would simply be the consistently-weighted penny, and the groat initially also held the very rough equivalent weight of 4d. This estimation would become increasingly less accurate throughout the 14th century as the groat became an increasingly light coin.

Things get more complicated when you introduce higher value coinages jnto the mix. The first real high value coinage in England is the mancus which appears intermittently from about the 7th Century onwards. The mancus was both an intermittently-minted largely ceremonial gold coin and a measure of value of gold of the same worth and the two terms were frequently used interchangeably. The value of a mancus was given specifically in its relation to its worth in silver pennies: the mancus was specifically 30d worth of gold. Since the mancus coinage was an uncommon and ceremonial issue, a 'mancus' also just became shorthand for a value of 30d regardless of whether or not gold was ever involved in the transaction. Extant examples of the mancus, of which there are very few, weighed in at 4.25g. Carrying "1 pound" in mancuses, therefore, would weigh only 34g.

The first really functional gold coinage that entered widespread use in England was the 'noble', introduced by Edward III in 1344. The noble was worth 6s 8d (80d, or one third of a pound). At its introduction, the noble weighed 9g, so carrying "1 pound" in nobles would weigh only 27g. As you might have gathered from the mancus, the weight of the gold coinage is tied to its value in silver. This of course means that, to keep the value of coinage as consistent as possible, the weight of gold coinage changed to represent fluctuations in the gold market. As you can see, a (silver) pound's worth of gold in the 9th or 11th century was 34g, while a silver pound's worth in 1344 was only 27g of gold. By 1346, the noble had decreased to 8.3g, and by 1351 it weighed 7.8g, so a 'pound' could be carried weighing just 21g.

Once you take nobles into account, therefore, determining the weight of a hypothetical purse simply through weighing it would be a lot more problematic. A purse that weighed 1lb could contain 240d worth of silver pennies, but if it contained nobles, that same purse could be worth as much as 20 pounds (4,800d). If it contained a mixture of coins then really it's anyone's guess.