There are a couple ways of looking at this.
I use a general rule-of-thumb which says to use cruising speed limits which are no more than 75 percent than the maximum speed when desiring long engine life.
There is another and much older rule called the "Pomeroy Dictum" which states that 2500 feet per minute average piston speed is the limit to remain under for good longevity on a street vehicle. This goes back more than 60 years, but many road vehicles still operate within that limit.
For the stroke length of the Enfield, the rpm would be about 4300 rpm to remain under the limits of the Pomeroy Dictum. I think for the typical UCE gearing, it would equate to somewhere around 65mph in top gear.