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The 92 should work with a Silverlink- odds are your cable has actually died (they're known to do that!) or you've set something up wrong (TI-connect is.. not a great piece of software).

If you're set on a new calculator (or not interesting in troubleshooting your Silverlink) my preference would be for the 89Ti since you'll find more and better resources for working with it due to its seniority and wider adoption. The Prizm is an okay device for math (though I don't really like its setup), but there seems to be less interest in programming it overall.

Depends on what languages you wish to learn - I love Casio Prizm for its support of addins written by users in C/C++, lcd resolution, cpu speed, storage memory, 3-pin connection - I wish there was a bit slimmer version but having aaa batteries power option is not bad still

Tari's right, you should be able to find more resources for programming the 89T than for programming the Prizm (fx-CG10/20/50). There used to be a sizable developer community for the TI-68k series (smaller than for the more popular TI-Z80 series); it largely died in 2006-2007, a couple years after the last OS update for the 89T and V200, and never picked back up.

Note that with that amount of money, you could by a Nspire CX CAS, which is effectively the 89T's newest successor, and take advantage of the color screen, higher computing power and newer toolchain. The Nspire series has never run assembly programs directly, but since late February 2010 (for the general public), Ndless always became available sooner or later after a new OS release.
The community of native code developers for the Nspire has belittled a lot.

89T for life. Forget the Prizm (even if it does look nice)

Either get a TI-84+CE or an Nspire. The CE is a better choice because it's slightly cheaper, it's color, and lots of people are developing for it. There's Basic, ICE, C, and ez80 to learn for the CE, starting with Basic, then ICE, and so on.

I find the Nspire CX/CX CAS very nice and it can be programmed in Python, JS, Lua, C, C++, TI-BASIC, ARM9 ASM and maybe more. It does need to be jail broken with ndless to program in these languages (Lua and TI-BASIC are the only ones that do not require it).

Still I doubt many courses will be taught with TI calculators in the UK. Everything seems to be dominated by Casio here - from what you can buy in shops to schools and universities.

It's hard to argue that the 84+CE's a better choice than the Nspire CX (CAS). The Nspire has much more computing power and storage space, and it uses a saner toolchain (standard ARM toolchain, whereas Zilog's broken C -> Z80/eZ80 compiler really sucks).