Grubbylabs,
Right now, milsurps from WWI due to its centennial and WWII are hot. The few being imported are in rough condition and prices are heading higher for the few well preserved rifles in original matching condition. $350 to $400 might have been the price in 2008 or so but now even pedestrian non-matching beaters and parts guns in well worn condition are getting that if they still have most of their military hardware and wood. If the rest of your rifle is as good as the pictures that you posted in metal and wood condition, then yours is quite a bit above the normal ones seen outside of specialty shops.
FWIW, the exterior of rifle appears in vg or better and from what I can see the wood is good on it. It also appears blued and has the earlier round cocking piece on the bolt. On the buttstock and under the wrist of the rifle, you will often see cartouches (impressed markings into the wood designating inspections and sometimes service/rearsenaling actions) and sometimes on the forearm depending on the length of service. You should also see a round brass disk secured with a screw on the right side of the buttstock (from behind). Markings on this indicate unit and sometimes mfg. A detractor from value would be the name of the importer stamped into the metal.
If you look on the left of the receiver, you should see a date and who manufactured it. It will also tell and whether your model number is the No. 1, Mk. 3 or is marked Mk. 3*. (In 1916 the British simplified mfg. by eliminating the magazine cutoff but some 1916's are Mk. 3 and some Mk. 3*).
Now, if your is an all original matching bolt, excellent bore, original bluing, vg or better condition, and is one of the early models of the SMLE, then I would expect more like $500 to $600 from a sale to individual collector (less to a dealer) or possibly more depending on exactly when it was made, rearsenaling, any additional items such as bayonets, slings, etc. and whether you have any idea of how and when your family acquired it.
One way to tell if you have a rare one is to buy the Stratton book on Amazon which goes into details about mfg. dates, parts identification, etc. which may help you establish whether you have a rare variant. See
http://www.amazon.com/British-Enfield-Internet-Workshop-collectors/dp/1882391160 It is about $20 on amazon.
BTW, unless you plan to testfire it, I would probably not bother checking the rifle's headspace as WWI SMLE's are known for having generous chambers and many gunsmiths today are unfamiliar how to measure it with Enfields and Krags because they headspace on the rim and not the shoulder. If you know one who has been in the business since the 60's and 70's, then more of them will know how to do it correctly.
If you must check it either to testfire or to get top dollar for it, buy a coin type gauge and employ a field gauge and a no go gauge-see
http://www.okiegauges.com/--a set will run you about $35 plus s/h. These do not require the removal of the extractor but follow the directions exactly. You can damage the gages and incorrectly measure headspace by forcing the bolt closed on a gage.
Removal of the extractor is necessary to use non-coin headspace gages, which is a pain without the proper tools. and most common gages sold are not mil-spec anyway. The Brit's did not reload their brass--e.g. Berdan primed so they really did not worry too much about generous chambers.
Sorry about the long post but good luck. BTW, if you have a rare variant, might want to check out milsurps.com forum or gunboards.com for truly expert opinions. Peter Laidler on milsurps is a particular expert along with Brian from BDL Ltd if I remember.
BTW, I am rebuilding an SMLE right now of WWI vintage which requires me to check dealers places like Simpsons or Liberty Tree Collectors for reference photos.