Labor was cheap and life was simpler. Inflation still happened. But while you can do a simple inflation calculator for 1907 vs today, It is hard to judge how much money that actually was. Compared to today, I would wager that 25 pounds was a painful amount of money for all but the very wealthy to separate with. You have to understand that a GBP was actually called a "pound sterling." which would indicate its value at the time it was introduced (in the middle ages). Now granted, a pound of sterling silver even in 1907 cost more than a single GBP, but the system was largely still based on precious metals. So the value was actually in the coinage which means that it didn't lose value the way that "paper gov't backed" bills do today.
Then you would need to know what the the average man spent everyday or every week on necessities. According to some research I just quickly did, most laborers in Great Britain at the turn of the 20th century made between 20 to 80 GBP per year depending on your skill set.
here are some of the facts about everyday expenses in 1906:
17 Shillings, 6 Pence - average weekly earnings for farm worker in 1906 (about 44 GBP/ yr.)
Rent - 1 shilling 6 pence
3 lbs. of Sugar - 5-1/2 pence
1/2 lb. of tea - 8 pence
1-1/2lbs. of butter - 1 shilling 6 pence
1 pint of beer - 2 pence
2 oz tobacco - 6 pence
So the cost of a good Jeffrey rifle back then cost the same as 750 pounds of loose tea. It is unfair to measure it against sugar because its cost fluctuates wildly until the middle of the 30s or 40s due to logistical and shipping costs as did a lot of the other things on that list, but tea is pretty stable because it is grown in the colonies, is simply farmed and dried.
so 750 lbs of tea in 1906 cost 25 GBP. In 2018, the cheapest I could find good English black tea was about $15 a pound. So that puts your Jeffrey rifle somewhere around $11,250. Still a steal, but according to that figure, which is likely not a true representation because tea is probably cheaper to make today because of machinery, the rifle was quite a bit more expensive. I would wager it was well out of the average working mans budget unless he used it to make a living, such as an African White Hunter. Probably somewhere in the $25,000-$50,000 range today. It is very hard to gauge.
Sorry that was kind of long winded. I really got into it and I love history.