Jersey

There are three carriers in Jersey.

JT (JT Global/Jersey Telecom):
2G: GSM 900/1800 [GPRS/EDGE]
3G: UMTS 2100 [W-CDMA/HSDPA] [1]

Airtel (Airtel Jersey/Airtel Vodafone):
2G: GSM 900/1800 [GPRS/EDGE]
3G: UMTS 2100 [W-CDMA/HSDPA] [1]

Sure (Cable and Wireless):
2G: GSM 900/1800 [GPRS/EDGE]
3G: UMTS 2100 [W-CDMA/HSDPA (contract only)] [1]

It looks like suitable modems for the current implementations will be:
PLS8-E, PHS8-P/J/K, PHS8-E or PXS8

PLS8-US looks like it would work, but not with UMTS.

There are no LTE carriers yet, but the regulator expects to have frequencies allocated this year. These will be in the 800 MHz and 2.6 GHz bands. [2]

This means that the ideal future proof modem is:
PLS8-E

Edit 13/02/2015:

The regulator allocated 800 MHz, 1.8 GHz and 2.6 GHz to all three carriers for LTE[-A] in July 2014. [3]

JT's LTE-A network is live as of February 2015. [4]

PLS8-E is therefore the correct modem, as expected.

[1] [2] [3] [4]

Edit 21/05/2015:

JT definitely appears to have an LTE-A (LTE Advanced) network (it appears in a wide range of marketing materials and FAQs). I can't find any evidence that Airtel or Sure do, but they do both seem to have at least LTE. Airtel claimed to wish to deploy LTE-A a couple of years ago [5].

The PLS8-E modem appears to be LTE only [6], but LTE and LTE-A are backwards and forwards compatible with each other so it is of no great concern, despite not being able to take full advantage of an LTE-A network.

[5] [6]